• Place a baking rack in the middle of the oven. Preheat oven to 325 degrees F. Lightly spray a 9- x 13-inch metal baking pan with cooking spray. Set aside prepared pan.

For the crust and topping:

• In the bowl of a stand mixer, on medium speed mix together the butter, white sugar, and powdered sugar until light and creamy.
• Add the vanilla and mix carefully until incorporated.
• Spoon the flour into a measuring cup, level with a straight edge utensil, and sift into the butter/sugar mixture. With mixer on low speed (or by hand), mix in the flour just until combined, forming a soft dough. Do not over-mix.
• Divide dough in half. Place one half of the dough, covered, in the refrigerator to chill. Press the remaining half of the dough evenly into the bottom of the prepared baking pan. Bake for 20 minutes, just until golden around the edges. Remove from oven and set aside to cool for 15 minutes.
• Turn oven up to 350 degrees F.

For the filling:

• In a large bowl, mix together the sugar, lemon zest, lemon juice, and cornstarch. Gently stir in the blueberries.
• Pour mixture onto baked crust. Spread evenly.
• Remove the chilled dough from the refrigerator. break or crumble the dough into small, chunky pieces and scatter randomly over the top of the blueberries.
• Bake for 30-35 minutes until top crust just barely starts to turn light golden brown. Remove from oven and allow to cool completely before cutting into 24-36 squares.