Preheat oven to 375 degrees F. Grease a 9x13-inch pan.
In the bowl of a food processor, add flour, baking powder, salt, 1 cup sugar, and lemon zest, and pulse briefly to mix. Add cold butter (cut into tablespoon-sized chunks) and the egg, and pulse until the mixture is crumbly yet sticks together. Alternatively, cut the butter and egg into the dry ingredients using a fork or a pastry blender. Pat half the dough into the prepared pan for the bottom crust.
In a medium bowl, stir together the sugar, cornstarch, and lemon juice. Gently mix in the blueberries. Sprinkle the blueberry mixture evenly over the crust.
Crumble the remaining dough on top of the blueberries.
Bake for 45 minutes or until top is slightly brown. Cool completely before cutting into squares. I typically cool the bars in the pan to room temperature then place the pan in the fridge overnight and cut them the next day. I think this is a bit easier.