Preheat your oven to 375°F (190°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal later.

In a large bowl, combine the flour, 1 cup sugar, baking powder, and salt.

Use a pastry cutter or your fingers to cut in the cold butter until the mixture resembles coarse crumbs, with pea-sized chunks of butter remaining.

Stir in the beaten egg and vanilla extract until the dough starts to clump together.

Press about 2/3 of the dough evenly into the bottom of your prepared pan to form the crust and place it in the freezer while preparing the filling.

In a separate bowl, combine the fresh blueberries, the remaining 1/2 cup sugar, cornstarch, lemon juice, and lemon zest. Gently toss to coat the berries.

Spread the blueberry mixture evenly over the chilled crust.

Crumble the remaining dough over the blueberry layer.

Bake in the preheated oven for 45 to 50 minutes, or until the top is lightly golden brown and the blueberry filling bubbles around the edges.

Allow the bars to cool completely in the pan before lifting them out using the parchment overhang. Cut into squares and serve.