Preheat oven to 350 F and grease a 9×13 baking dish.

Berry Center-

In a saucepan combine 2 cups of the blueberries, strawberries, lemon juice, sugar, and cornstarch. Turn heat to medium low and stir frequently till blueberries and strawberries have lost lots of their water and the mixture is really wet, about 6 minutes. Increase heat to medium and bring to a simmer. Stirring constantly, let mixture simmer for 2 minutes. Remove from heat and stir in remaining 1 cup blueberries, butter, and vanilla till butter is melted and combined. Let sit and cool a bit while you make the crumble layers

Crumble layers-

Combine the flour, sugar, brown sugar, baking powder, salt, and cornstarch. Add the butter and using a pastry blender, cut the butter in till the butter is about the size of a small pea. Add the egg yolks and vanilla extract and continue using the pastry blender to blend till the mixture is combined. It will start to somewhat form a dough but will still be crumbly.

Assembly-

Press roughly 1/2 -2/3 the crumble mixture into a greased 9×13 baking dish with your hand till it is formed into an even flat layer. Pour the berry layer next and smooth till even. Sprinkle the remaining crumble layer over the berry layer and place in oven.

Bake for 45-55 minutes till topping is golden brown.