Blueberry Crumble Bars

Preheat the oven to 350F (180C) degrees. Line an 8x8 inch square baking pan with aluminium foil or parchment paper so that there's an overhang around the edges. 

In a medium bowl toss together the ingredients for the blueberry layer (blueberries, cornstarch, lemon zest, lemon juice & sugar). Set aside.

In a separate large bowl whisk together the 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, baking powder and salt.

Cut the cubed butter and egg yolk into the flour mixture using a pastry cutter or 2 forks (a pastry cutter will be easier). When finished, the mixture will be slightly crumbly, but

Reserve 1 cup of the butter mixture for the crumb topping. Press the remaining shortbread mixture into the prepared pan to form the base layer of the bars.

Spoon the blueberry mixture over top of the shortbread base. You want all of the shortbread to be covered with an even layer of berries. If there's a lot of liquid at the bottom of the bowl (this is more likely to be the case if using frozen berries) do not add the excess liquid to the pan.

Add the 1 cup of reserved shortbread/crumble mixture back to the bowl. Add in the 1 tablespoon of all-purpose flour, 1 tablespoon of granulated sugar and 1 tablespoon of brown sugar. Use a fork or pastry cutter to incorporate the additional ingredients into the mixture. It will be even crumblier.

Crumble the remaining dough over the blueberry layer by squeezing it in a fist with your hand, then crumbling it over top.

Bake in the oven for about 40 minutes, until you see the blueberries bubbling and the crumble layer starting to turn golden. If you used frozen berries, it may need a bit longer to bake.

Cool bars fully until the pan is room temperature (about 3-4 hours). Then lift the bars out of the pan using the overhang of the parchment paper/aluminum foil. Place on a cutting board and slice using a sharp knife.