First, zest the lemon and set aside the zest. In a medium saucepan combine the berries first with the sugar and cornstarch and then juice of the lemon. Stir to coat. Place over medium heat and stir occasionally with a wooden spoon or rubber spatula. After about 5 to 8 minutes, the mixture will start to steam and bubble. Stir continuously until the mixture is thickened and glossy, like pie filling.
Remove from heat and mix in the lemon zest and vanilla. Pour into a wide and shallow dish and allow to cool in the refrigerator while you make the crumble.
Crumble
Preheat the oven to 325 and grease the inside of a 9×13 baking dish. Line the bottom and all sides with parchment paper.
Add the brown sugar, sugar, flour, salt and baking powder to a large bowl. Use your hands to mix and toss together, breaking up any brown sugar lumps as you go.
Use a rubber spatula to stir in the butter. Spend a good couple minutes on this to ensure the mixture is evenly hydrated and no dry patches remain. It should look moist but very crumbly.
Scoop about ⅔ of the crumble into the pan and press into a thin and even layer all the way to the edges.
Spread the berry filling on top, also in a thin and even layer. Then sprinkle the remaining crumble on top, taking care to ensure the berry is evenly covered.
Bake for 30 to 40 minutes or until lightly golden on top.
Allow to cool at room temperature for about 15 minutes, then transfer to the refrigerator to cool completely, about 45 minutes.
Then remove from the pan and slice into 12 large bars or 24 regular bars. Enjoy!
