Preheat oven to 350 degrees. Line a 9 or 10-inch square baking dish with foil (overhanging edges for easy lifting out of pan to slice) and spray with cooking spray. Set aside.

In the bowl of stand mixer, combine flour, sugar, salt and lemon zest. Mix on low speed until well combined. Keep mixer on low speed and add cold butter, one piece at a time, until texture resembles that of wet sand and butter is very well incorporated.

Stop mixer and reserve 3/4 c. of the mixture for the topping... place this in the refrigerator until you are ready for it.

Press the rest of the mixture gently and evenly over the bottom of the pan. Bake for 10-12 minutes, remove from oven and allow to cool while mixing the filling.

In a large bowl, combine the eggs, sugar, sour cream, flour, salt, lemon juice and vanilla. Whisk until very well combined. Gently fold in frozen blackberries.

Pour the filling evenly into the pan over the prepared crust. Evenly crumble the reserved topping mixture over the filling.

Bake on center rack for 45-50 minutes, or until filling is just set. Allow to cool completely on a wire rack and chill in the refrigerator a bit before slicing and serving.