Preheat oven to 350 degrees. Line a 9x13 inch baking sheet with parchment paper or spray with nonstick cooking spray.

Using a food processor, pulse sugar, baking powder, salt, and flour until mixed.

Add cold butter slices and pulse until small pea sized clumps form.

Add egg and ½ of lemon zest to dough and pulse until combined. Dough will be crumbly.

If you do not have a food processor, sift together sugar, baking powder, salt, and flour. Using a pastry cutter, cut in butter to flour mixture. Add egg and half of lemon zest until you have a crumbly dough.

Press half of dough into the pan.

Mix blackberries with lemon juice, sugar, and cornstarch.

Sprinkle blackberry mixture over bottom crust.

With remaining dough, sprinkle over blackberry mixture.

Bake for 45 minutes, or until top has slightly browned.

Cool for 10-15 minutes then chill for 1 hour (not obligatory but recommended) before cutting into bars.