• Preheat your oven to 375℉. Line a 9x13-inch tray with parchment paper.
• Wash your blueberries and pat dry. Cut butter into small cubes and set aside.
• Add the flour, brown sugar, granulated sugar, baking powder and salt in a large mixing bowl and whisk together. Set aside.
• In a separate bowl, whisk together the egg and vanilla extract.
• Add the egg and vanilla mixture and butter cubes into the large mixing bowl with dry ingredients. Use forks, pastry cutter or your hands to bring the mixture together until it looks like large crumbs. You don't need to knead this into a firm ball of dough.
• Press 1/2 of the mixture into the baking tray. (Half of the mixture is about 428 grams.)

Blueberry Filling

• In a mixing bowl, whisk together the remaining 1/2 cup granulated sugar, lemon juice and cornstarch. Add blueberries to the bowl and evenly coat them in the mixture using a spoon.
• Pour the blueberry mixture over the cookie dough in the baking tray, spreading them out evenly.
• Take the leftover cookie mixture and sprinkle it over the blueberries. This doesn't have to be perfect, just crumble the bigger pieces in-between your hands.
• Bake for 35 - 40 minutes until the top looks golden brown.  
• Cool the bars for 1-2 hours before cutting into 15 medium sized bars.