• The lemon curd can be made up to a week before cake assembly. Store it in the fridge in an airtight container. You won't need the full curd recipe for the cake, just about 1/4 cup of it so you can halve the recipe and have a bit of leftovers for other uses.

To make the cake

• Butter and flour two eight inch round metal cake pans. Preheat the oven to 325 F.
• Set the buttermilk in a small pot and stir in the poppy seeds. Set the pot over low heat. Once the mixture is warm, remove from heat and set aside.
• In the bowl of a stand mixer add the sugar and over it zest the lemons. Rub the zest into the sugar with your fingers to allow the oils to release. Stop once it resembles wet sand.
• Add the butter, oil, vanilla and salt to the bowl and affix the paddle attachment. Cream the mixture together until light and airy, about 3-5 minutes at medium speed. Stop the mixer and scrape down the bowl to ensure no bits remain unmixed.
• With the mixer on, add the eggs one by one. Once they are all in, turn the speed to medium and beat until you have a creamy mixture.
• Sift in the flour and baking powder and with the mixer on, pour in the buttermilk poppy seed mix. Mix to just combine.
• Divide the batter evenly between the two pans and smooth the batter so it’s in one layer.
• Bake the cakes until a cake tester comes out clean, the tops spring back - about 30-35 minutes. Let cool briefly in the pan then invert onto a cooling rack to completely cool.

To make the buttercream

• Make sure the butter is at a soft room temperature before you begin.
• Find a pot that will hold a glass bowl over it. Fill the pot a third of the way with water and set it over medium low heat and bring it to a simmer.
• In the glass bowl (off the heat) add the sugar and whisk in the egg whites. Once combined, set the bowl over the simmering water. Stir frequently with a rubber spatula to scrape up all the sugar on the sides as it warms. Once you can pinch the mixture and there are no sugar granules, it can be taken off the heat.
• Set the mixture in the bowl of a dry and clean stand mixer. Affix the whisk attachment and begin whisking on low, add the cream of tartar as it whips.
• Whip to stiff peaks, this can take up to 10 minutes but you’re looking for a meringue that holds shape (take the whisk out and turn it up - if the meringue droops it needs more time whipping).
• Add the vanilla and salt (and lemon extract if using) and then, with the mixer on, slowly add the butter - one tablespoon at a time. You don’t need to wait for each tb of butter to be mixed in but add it slowly. Once all the butter is in the buttercream might look soupy and broken, keep whipping until it thickens. When you can swipe it with a finger or an offset spatula and find it smooth, it’s done. Stir in the poppy seeds.

Assemble the cake

• If the cakes have domed, slice of the tops so you have even layers.
• Set one cake, facing up on a cake turntable. Pipe a buttercream border around the cake. In the center, spread the lemon curd (or spread more buttercream if that’s what you’re using).
• Set the second cake face down on top of the cake and pipe buttercream along the sides, turning the cake as you do. Smooth the buttercream in an even layer, using a bench scraper held against the sides as you turn the cake. Spread buttercream in an even layer, with some swoops, over the top of the cake.