Preheat oven to 375 degrees. Butter a 13 by 9-inch baking dish (also line with parchment and butter parchment if you'd like to lift them out easier), set aside.
In a large mixing bowl whisk together flour, 1/2 cup granulated sugar, brown sugar, baking powder and salt.
In a small mixing bowl, whisk together egg and vanilla until combine.
Add egg mixture and cold butter cubes to flour mixture and using a fork or a pastry cutter, cut mixture together until it resembles coarse crumbs.
Press 1/2 of the mixture evenly into prepared baking dish.
In a separate mixing bowl, whisk together remaining 1/2 cup granulated sugar and cornstarch, stir in lemon juice.
Add blueberries then toss mixture to evenly coat, pour and spread coated blueberries evenly over crust in baking dish.
Sprinkle remaining crumb mixture over berry layer.
Bake in preheated oven about 38 - 42 minutes until top is golden brown.
Allow to cool for about 20 - 30 minutes before cutting into squares.
Store cooled bars in an airtight container in refrigerator (rewarm in microwave for 10 - 15 seconds if desired).