Preheat oven to 375 degrees F and spray a 9x13 dish with non stick cooking spray.

In a medium bowl, mix together the sugar, baking powder, flour, salt and lemon zest.

In a separate small bowl, whisk together the egg and vanilla until well combined.

Using a food processor, blend the flour mixture, the egg and vanilla mixture and the butter until dough is a crumbly consistency.

Sprinkle half of the dough in the greased pan evenly and pat it down to form the crust.

In a large bowl, mix the sugar and cornstarch until combined. Then gently stir in the blackberries.

Pour the blackberry mixture evenly over the crust and then sprinkle with the remaining dough.

Bake for 45 minutes or until top is just browned.

Cool completely and cut into squares.