Lemon Blackberry Cake
Two layers of plush lemon cake studded with fresh blackberries and sandwiched between two layers of an italian meringue buttercream made with fresh blackberry puree.
This recipe for lemon blackberry cake wasn’t quite on the agenda for fall but serendipitously it is here just as the berry season is coming to a close. We went berry picking and, while I was there for the raspberries, my family (reluctant to reach through thorns and bees for those tiny red berries) enthusiastically picked way, way too many blackberries which were tantalizingly hanging off the tall bushes like grapes from the heavens. They didn’t get to eating them all before the berries started going soft, so this cake was born!
Recipe Origins
I went through all my cake recipes to see which would be the best fit: I wanted a cake without too strong of a flavor (so, not chocolate) that would complement and allow the blackberry flavor to come through (I considered brown butter, then scraped it – I’ve done so much of that lately).
EBC was a first candidate but the lemon version of her sounded like it would work so much better, the added zest and citrus bringing the bright berry flavor into the spotlight. Rather than make the cake as is then attempt to slice it with those large berries stopping the path of the knife, I kept the square shape but baked it into two separate layers, berries arranged so it would slice easily.
The frosting is something truly wondrous: it is on my agenda to bring you a recipe I developed awhile ago for an apple cider meringue buttercream, and I simply thought, why not use fresh blackberry juice like I do in that one? It’s a bit more effort than the cake as you’ll have to fuss over the sieving part, then cooking to candy point (245-248 F), then whipping egg whites, and lastly, slowly adding the butter as it whips.
But it is absolutely, 100% worth it. So creamy and not overly sweet, plus there’s that popping blackberry flavor right there =)
Recipe Ingredients
For the cake
Sugar: fine granulated sugar. Don’t reduce it, this can impact the moisture of the cake.
Lemons: organic, if using conventional, wash and rub the skin to remove any wax. We’re using the zest for the cake (as much as possible!) and some of the juice for the cake too.
Butter: unsalted and softened to room temperature. The butter should indent easily when pressed.
Salt: fine sea salt. Please note, if using table salt reduce the salt added as it has a saltier taste.
Vanilla: pure vanilla extract.
Eggs: large eggs, it is essential they are at room temperature or the batter will split (see below for what to do if the batter splits).
Cake flour: with its lower protein content, finer mill and as it is made from a softer wheat, cake flour will give us the ideal crumb: wonderfully light and tender. Pastry flour can be a substitute but remove two tablespoons of flour and add in two tablespoons of tapioca or cornstarch. If you absolutely must use all purpose, do the same with the tablespoon of starch swap and consider the cake won’t be as soft and plush.
Baking powder: not baking soda (which has the power of 4x baking powder).
Sour cream: full fat. It does not have to be at room temperature. If you need to substitute, use labneh. Greek yogurt works too, it should be full fat and consider that the crumb will be a bit more open.
Optional: lemon extract, for a stronger lemon flavor in the cake. Add half a teaspoon.
Blackberries: it is best to use fresh. Often using frozen means there is added water to the batter, around the berry as the ice melts. Some of the blackberries will be used in the cake, some will be pureed into a juice for the buttercream, and some to be used to decorate the top.
For the buttercream
Sugar: fine granulated sugar. Don’t reduce it, this can impact the structure of the meringue.
Blackberry puree: we’re going to process some of the fresh blackberries into a puree, then sieve them to get a blackberry juice. This is what we’ll use to make the sugar syrup.
Egg whites: there should be absolutely no egg yolk mixed into the whites. Fat, like from egg yolks or from a dirty bowl/utensil, will inhibit the egg proteins from forming their tight structure which makes the meringue.
Cream of Tartar: this helps stabilize the egg white foam as it is being whipped.
Butter: unsalted butter. Salted is ok, though you’ll probably want to skip adding any additional salt unless you taste the buttercream and find it lacking. In which case, add a pinch and test. Repeat as needed.
Vanilla: pure extract or vanilla bean paste.
Salt: fine sea salt. Avoid table salt.
How to make Lemon Blackberry Cake
Preheat the oven and flour and grease two 8×8 square metal pans (light colored – darker pans will cause more browning).
Zest the lemons over the sugar and rub together. This releases the oils.
Add the softened butter and beat until creamy and airy.
Add the salt and vanilla while beating.
With the mixer on, one by one, add the room temperature eggs. Whip until airy and fluffy.
Sift in the cake flour and add the baking powder.
With the mixer on, add the sour cream and lemon juice. Mix to just combine.
Divide the batter evenly between the two prepared pans and smooth it into an even layer.
Arrange 16 fresh blackberries in lines of four, according to how the cake will be sliced (see photo, or video for guidance).
Bake the cakes until the middle springs back when pressed.
Let cool on a cooling rack completely before frosting (in some of the photos you can see I added the middle buttercream layer before the cake was fully cool which melted it slightly).
How to make Blackberry Buttercream
In a food processor, puree the blackberries. Press the puree through a strainer to remove the seeds. You need 80g or ⅓ cup for the sugar syrup.
In a pot, add the sugar and blackberry juice and stir. Set over medium high heat.
Whisk until the sugar dissolves then keep cooking, stirring occasionally. Use a candy thermometer, an instant read, or a laser thermometer to keep an eye on the temperature – we aim to candy it, so it must reach 245 F.
When the sugar syrup is getting close, add the egg whites to a clean and dry mixing bowl and affix the whisk attachment.
Add the cream of tartar and whip until frothy, about 3-4 minutes.
When the sugar has reached 245 F, turn the mixer down to low and slowly pour in the syrup as it whisks.
Once all the syrup is in, whisk until the meringue reaches stiff peaks. It should hold shape when the whisk is lifted and held meringue side up.
Turn the mixer back on and add the salt and vanilla, then the butter. Add one pat at a time (about a tablespoon).
Add the butter slowly – the meringue will look like it’s turned into a soup but once all the butter is in, keep whisking on medium high until it starts to thicken. Use a spoon to check to see if it’s smooth by pressing and swooping it into the buttercream.
Lay one cake on a serving plate and spread half of the buttercream on top. Optional: zest a lemon over the buttercream.
Place the other cake on top and spread the rest of the buttercream on top.
*If you are using two round 8 inch cake pans, you should have enough to frost the sides of the cake as well though you’ll want to make the frosting layers thinner.
Decorate with fresh berries and edible flowers, if desired.
Read Ahead: Common Mistakes & Troubleshooting
My cake batter split after I added the eggs. What happened and what can I do?
The eggs and/or the butter were too cold and caused the butter to clump, ideally you have everything at a warm room temperature before making the batter.
If it splits, you can take out a few spoonfuls of batter and warm it – just enough for some of the butter to melt and then add it back into the bowl. Then whip, it should come back together as the heat from the melted butter softens the rest of the butter in the batter.
Some of the blackberry sugar syrup has congealed and is not mixing into the egg whites.
This happens when the syrup is added too quickly or the mixer is on too high a speed (you can see it congealed around the sides of the bowl in the instructional video below). To fix it, take a rubber spatula and scrape it off the bowl and put it back into the middle. Usually the mixer can work it into the meringue, if not – don’t worry about it as long as you have enough syrup in there to hold the meringue.
My meringue isn’t reaching stiff peaks.
Sometimes patience is key, but if it’s been a good 15 minutes and you aren’t getting anywhere close to a stiff meringue it’s possible that some fat (butter, egg yolk, oils from utensils or the bowl) got into the mixture and prohibited the structure from developing.
The buttercream split after adding the butter to the meringue.
This will always happen, however with some strong whipping it should come back together. If it isn’t thickening, it’s usually a temperature issue: if the butter was too warm then you’ll notice it as overly soupy. This is an easy fix: set the bowl (keep the whisk in there so you don’t have to clean it) in the fridge for 5-10 minutes, then try again.
If the butter simply isn’t mixing into the meringue, it might be too cold. You’ll need to warm it. Set it over or in something hot (the oven, or over a pot of steaming water) to warm it and then try whipping again.
Lemon Blackberry Cake Recipe
Lemon Blackberry Cake
Ingredients
Lemon Cake with Blackberries
- 250g or 1 ¼ cups fine granulated sugar
- Zest of 3-4 lemons
- 226g or 1 cup unsalted butter at room temperature
- 3 large eggs at room temperature
- 1 teaspoon pure vanilla extract
- ½ teaspoon fine sea salt
- 260g or 2 cups cake flour
- 2 ¼ teaspoons baking powder
- 120g or ½ cup full fat sour cream
- 60g or ¼ cup freshly squeezed lemon juice
- About 510g or 18 oz fresh blackberries some for the cake, 200g for the puree (below) and some leftover to decorate
Blackberry Meringue Buttercream
- 200g fresh blackberries about two cups, to process into 80g or ⅓ cup blackberry puree
- 200g or 1 cup fine granulated sugar
- 3 egg whites from 3 large eggs - no yolk mixed in at all
- ¼ teaspoon cream of tartar to stabilize the meringue
- ¼ teaspoon fine sea salt or more to taste
- 1 teaspoon pure vanilla extract
- 254g or 1 cup plus 2 tablespoons butter unsalted and at room temperature (if using salted butter, you may want to skip adding the salt)
Method
Make the cake
- Important: have your butter at a warm room temperature and your eggs at room temperature. For the eggs, you can place them in a bowl of hot water to warm them.
- Grease and flour two 8 inch square (round works too) light colored metal pans. Preheat the oven to 350 F.
- Into the bowl of a stand mixer, add the sugar. Zest the lemons over the sugar then use your fingers to rub the zest into the sugar to release the oils.
- Add the butter, salt and vanilla and affix the paddle attachment. Turn the speed to medium and beat until creamed, about 3-5 minutes.
- With the mixer on, and one by one, add the eggs. Allow 30 seconds between each egg addition. Once all three eggs are in, mix on medium speed until light and airy. (If the batter splits at this point, scroll above to read what to do).
- Sift in the cake flour and add the baking powder. With the mixer on low, add the sour cream and fresh lemon juice. Mix until just combined - scraping down the bowl as needed to ensure an even mix.
- Divide the batter between the two prepared pans, smooth the batter into an even layer. Arrange the blackberries on top, 16 berries per pan in rows of four (adding them on top means there will be slight sinking in the top of the cake, but adding them into the cake would have them all sinking to the bottom which we want to avoid).
- Bake the cakes until the tops spring back when pressed in the middle (of the cake, not the berry), or a cake tester comes out clean, about 15-18 minutes.
- Once the cakes cool slightly, carefully flip them over to release and transfer to a cooling rack to cool completely.
Make the buttercream
- Add 200g of blackberries to a food processor and process until pureed. Press the berries through a fine mesh sieve to remove the seeds. We’ll want about 80g or ⅓ cup of blackberry juice for the syrup. You may end up with more than needed, depending on how juicy the blackberries are.
- In a pot, add the sugar and 80g of blackberry juice, stir to combine and set over medium heat. Cook as the sugar dissolves and the mix begins to boil. Keep a thermometer handy, we’ll take it off the heat when it reaches 245 F (a little above is ok, don’t let it get over 250 F).
- When the syrup is close to being done, add the egg whites to a clean and dry stand mixer bowl. Affix the whisk attachment and add the cream of tartar. Turn the mixer on low, whipping until the egg whites are frothy.
- Once the sugar reaches 245 F, remove it from the heat and with the mixer on low slowly pour it onto the frothy egg whites. Scrape the pan to ensure all the syrup is in the bowl and turn the mixer to medium speed. If you see lots of syrup stuck to the sides of the bowl, stop the mixer and scrape it into the middle then continue whisking.
- Whisk until you have stiff peaks, the meringue should hold shape when the whisk is turned upside down. Add the salt and vanilla and whisk on low.
- Slowly add the butter, one tablespoon or pat at a time. The buttercream will split and look soupy, continue adding the buttercream. Once it’s all in, keep whisking until the buttercream is thickened and smooth (if it does not thicken, see above section on troubleshooting for what to do).
Assemble the cake
- Lay one cake onto a serving plate and spread half the buttercream on top. Place the other cake on top, and spread the rest of the buttercream over it. Decorate with fresh berries and/or edible flowers.
- Cake is best day of but leftovers can be stored in the fridge. Allow them to come to room temperature so the buttercream will soften before eating.
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