Preheat the oven to 375°F (190°C). Grease the bottom and sides of a 9x13-inch glass or light-colored metal baking dish. Line the dish with a sheet of parchment paper, allowing it to extend over the two shorter sides for easy removal after baking. Grease the parchment lightly as well.

In the bowl of a stand mixer fitted with the paddle attachment, combine 1 cup granulated sugar, flour, and baking powder. Add the salt and lemon zest, mixing on low speed until fully incorporated. Add the cold cubed butter and mix until the texture resembles coarse crumbs with small pea-sized pieces of butter remaining. Add the egg and mix on medium-low speed until the dough forms a crumbly consistency.

Transfer approximately half of the dough to the prepared baking dish. Press it evenly into the bottom of the pan to form a solid crust layer. Set the remaining dough aside for use as the topping.

In a separate bowl, stir together the remaining ½ cup granulated sugar, cornstarch, and lemon juice until well combined. Add the blueberries and gently mix until the fruit is evenly coated with the sugar mixture.

Evenly distribute the blueberry filling over the crust in the baking dish. Sprinkle the reserved dough over the berry layer, allowing small portions of the fruit to remain visible.

Place the dish in the preheated oven and bake for approximately 45 minutes, or until the top is lightly golden and the filling begins to bubble at the edges.

Remove the baking dish from the oven and allow the bars to cool completely on a wire rack. Once cooled, transfer the dish to the refrigerator and chill thoroughly before slicing into bars.