In the bowl of a food processor, add the flour, sugar, baking powder, salt, and lemon zest. Pulse a few times to combine.

Add the butter pieces to the flour mixture; pulse 10-12 times, until the mixture resembles coarse meal with some pea-sized pieces of butter throughout.

In a small bowl, whisk together the egg and vanilla. With the food processor running, stream in the egg mixture. Let the food processor run for a couple of seconds after the egg has all been added to finish incorporating. The dough will be very crumbly, but should hold together when you squeeze it in your hand.

Transfer just over half of the dough mixture to the prepared baking pan. Use your hands and/or the bottom of a glass or measuring cup to press the dough into an even layer in the bottom of the pan. Bake for 10 minutes, or until the edges are golden and the center is just set.

In a large bowl, stir together the blueberries, sugar, and cornstarch. Spread this mixture evenly over the baked crust. Sprinkle the remaining dough evenly over the blueberries; for larger crumbs, squeeze some of the dough together in your hands as you add it to the pan.

Bake for 35-40 minutes, or until the top is lightly golden and the fruit is bubbly around the edges. Allow to cool completely in the pan before cutting into squares and serving.