Preheat the oven to 375 degrees. Coat a 9 x 13 glass baking dish with cooking spray.
In a medium bowl, whisk together flour, brown sugar, ½ cup white sugar, baking powder, salt, and lemon zest. Use a fork or pastry cutter to cut in the butter until the mixture is in pieces no larger than a pea.
Mix in the egg and vanilla. The dough will be crumbly but will stick together when squeezed.
Press half of the dough into the prepared baking dish.
In another small bowl, mix white sugar, lemon juice, and cornstarch until well combined. Add blueberries and gently toss them to coat.
Pour the blueberry mixture over the dough in the pan.
Then crumble the remaining dough evenly over the blueberries.
Bake in the oven for 50 to 60 minutes, or until the top is slightly browned.