Blueberry Rhubarb Bars

Buttery sweet shortbread bars with a layer of tart blueberry rhubarb jam in between. 

This recipe is inspired by two of my favorites: some raspberry shortbread bars from joy of baking (I made those once in college and they left a mark on everyone) and Ina’s blueberry rhubarb jam. You’ll notice lots of changes: I use a combination of sugars for the shortbread, finding this gives them more of that traditional shortbread texture, I upped the salt and advise lemon zest to give them more flavor. For the jam, I do without the apple (I didn’t want the taste of apple coming through into the jam so it would be pure blueberry and rhubarb) and use tapioca starch as a thickener, which is more suitable for bars. 

Ingredients for Blueberry Rhubarb Bars  

Rhubarb: About four stalks if you are making the full amount of jam (you only need half for the recipe, the rest keeps well in the fridge.) It’s okay if they are a bit soft, be sure to cut off the ends. Color doesn’t matter, rhubarb whether green or red tastes the same. 

Blueberries: Fresh or frozen is fine, but make sure there’s not too much ice buildup on them if frozen. And if fresh, get ones that taste good! A lot of out of season berries are quite bitter and that will come through in the bars. 

Tapioca starch: A thickener that will make the jam cohesive and silky. I keep this on hand more often than cornstarch (it has no chalky taste and will keep the filling clear rather than cloudy) but you can use cornstarch if that’s what you have. 

Lemon: Zest for the shortbread and the juice will go into the jam, helps give the shortbread a sweet flavor and brings out the rhubarb taste in the jam. 

All-Purpose Flour: Any all purpose will do. If you want to make these gluten-free, use a 1-to-1 substitute. 

Butter: Softened and unsalted; the former so that it will blend well into the shortbread, the later so that we can control the amount of salt. 

Fine Sea Salt: if using table salt, reduce by half the amount. 

Powdered Sugar: If organic (which I tend to prefer, esp that with tapioca instead of cornstarch), be sure to sift it. 

How to make Blueberry Rhubarb Jam 

First, the jam: 

Everything into a pot (be sure to dice the rhubarb so it cooks with the blueberries) and cook on low until the fruits are completely soft and the mixture is thick, at least 30 minutes. 

Second, the shortbread: 

Everything into a stand mixer (or bowl, and use a hand mixer) and blend until the dough is mostly cohesive.

Layer & bake: 

Most of the shortbread goes into the bottom of the pan. Make sure the pan is well-prepared with two sheets of parchment paper to make a + so that you have overhang on all sides (this will make lifting the bars out much easier). 

Press ⅔ of the shortbread into the pan, pack it in as best you can, using the bottom of a measuring cup (dust it with flour or powdered sugar so the dough doesn’t stick to it) or an offset spatula to even out the layer. 

Then spread the jam – if you’ve made half the jam, spread all of it. If you made the full recipe below, spread half. 

Lastly, the remaining shortbread: with this just drop small pinches of the dough on top, spreading them out so they are all over the jam. 

Bake the bars: doneness is a little difficult with these but keep an eye on the top shortbread layer, once it begins to brown the bars are almost done. 

Let them cool. These slice miraculously easily, even at room temperature. 

Can you freeze these blueberry rhubarb bars? 

Yes, for months! Keep them in an airtight container. 

What can I use if I can’t find rhubarb? 

I think pomegranate would be a great substitute as it’s got the same tart taste. However, the bars will have a lot more seeds! Pomegranate seeds are edible of course but if you don’t like the idea of a seedy jam perhaps use raspberries instead. 

Can I make these dairy-free rhubarb bars? 

Yep, use dairy-free butter. No other adjustments necessary. 

Can I double this recipe? 

Yes! The jam recipe can be kept as is (I intentionally make a double batch so I can have some actual jam on hand afterwards!) but the shortbread recipe will need to be doubled. Bake it in a 13×9 cake sheet pan and extend baking time by at least 10 minutes but judge doneness by how the bars look. The shortbread bits on top are a good indicator of doneness; when they turn golden the bars probably done.

A delicious suggestion: If you’d like to add a lovely note of lemon to the shortbread, zest the lemon you’ll juice for the jam into the granulated sugar. Kept in an airtight container, it can wait in the fridge overnight before you make the dough. 

Blueberry Rhubarb Bars Recipe

Blueberry Rhubarb Bars

Buttery sweet shortbread bars with a layer of tart blueberry rhubarb jam in between.

Prep Time: 20 mins

Cook Time: 1 hr

Yields: 12

5 from 2 votes

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Ingredients

Blueberry Rhubarb Layer* (makes double what you need for this recipe, halve it if you don’t want extra)

• 220 g granulated sugar about 1 cup
• 280 g fresh rhubarb diced, about 4 stalks
• 310 g blueberries 1 pint
• Juice of 1 lemon
• 2 tablespoons tapioca starch can substitute with cornstarch

Shortbread Layer

• 226 g unsalted butter, softened, 1 cup
• ¾ tsp fine sea salt
• 2 teaspoons pure vanilla extract
• 30 g powdered sugar ¼ cup
• 50 g granulated sugar ¼ cup
• 260 g all purpose flour 2 cups

Method

First make the jam:

• Combine all ingredients in a pot and set over low heat. Cook for about 30 minutes as the blueberries burst and the rhubarb softens. Stirring often and use the back of a wooden spoon to press down the rhubarb. Cook until the consistency is thick like jam and the fruits are almost completely mushed. Set aside. This can be made ahead of time (it’ll keep in the fridge in an airtight container for about 2-3 days).

Make the shortbread:

• Preheat the oven to 350 F. Line a greased 8 or 9” square pan with parchment paper so you have overhang on all sides.
• Beat together the butter, salt and vanilla. Add the sugars and the flour and beat to combine.
• Press ⅔ of the shortbread mixture into the bottom of the pan. Use a measuring cup or offset spatula to even out the dough. Spread half of the jam over the shortbread. Sprinkle the remaining shortbread over the jam in bits.
• Bake for 30-35 minutes until the shortbread on top turns golden. Allow to cool in the pan, then slice.
• Leftovers can be frozen, kept in an airtight container.

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