Line a 9-inch by 9-inch metal baking pan with parchment letting the parchment hang over two opposite sides of the pan. Spray the parchment with nonstick cooking spray, especially on the sides of the pan.
To Make the Crust
Whisk flour, sugar and salt together in a large bowl.
Mix in butter and cut it in with your fingertips until the butter is covered in flour and the size of peas.
Add in egg and vanilla extract until combined, but still crumbly.
Transfer all but 1 cup of loosely packed dough into the parchment-lined baking pan. Firmly press the dough into an even layer in the pan.
Whisk cornstarch, sugar, lemon zest and salt together in a large bowl.
Add remaining ingredients and stir to coat.
Pour the berry mixture onto the dough and spread into an even layer.
Crumble the reserved dough over the berries.
Bake on the bottom rack of the oven until the filling is bubbly and the topping is lightly browned, about 45 minutes minutes.
Allow bars to cool for 1 hour and pop into the fridge for 1 hour to make them easier to cut. Cut into bars and serve.