Preheat oven to 375°F (190°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang.

In a large bowl, whisk flour, ¾ cup granulated sugar, light brown sugar, baking powder, and ½ teaspoon salt. Add cold, cubed butter and cut it in until coarse crumbs (pea to lentil-sized) form.

In a small separate bowl, lightly whisk the cold egg with 1 teaspoon vanilla extract. Create a well in the butter-flour mixture; pour in the egg mixture. Gently mix until a shaggy dough forms, being careful not to overmix.

Reserve about 1 ½ cups of this dough for the crumble topping; refrigerate. Press the remaining dough evenly into the bottom of the prepared pan to form the base.

Pre-bake the crust for 15-18 minutes, or until lightly golden around the edges. Remove from oven and cool slightly, keeping the oven on.

In a medium saucepan, combine fresh or frozen blueberries with lemon juice.

In a separate small bowl, whisk together ½ cup granulated sugar, cornstarch (or tapioca starch), a pinch of salt, and optional lemon zest. Sprinkle this mixture over the blueberries in the saucepan; stir gently to coat.

Place the saucepan over medium heat. Bring to a gentle simmer, stirring frequently. Cook for 5-7 minutes until the filling is thick and glossy, and the blueberries have softened slightly. Remove from heat and stir in ½ teaspoon vanilla extract.

Allow the blueberry filling to cool for about 10-15 minutes to thicken further.

Carefully pour the slightly cooled blueberry filling evenly over the warm, pre-baked cookie crust in the pan.

Retrieve the reserved cold crumble dough from the refrigerator. Generously sprinkle it evenly over the blueberry filling; do not press down.

Return the pan to the preheated 375°F (190°C) oven. Bake for an additional 25-30 minutes, or until the crumble topping is beautifully golden brown and the blueberry filling is bubbly around the edges.

Remove from oven and place on a wire rack to cool completely at room temperature for at least 2-3 hours. For the cleanest cuts, chill in the refrigerator for at least 1 hour after it has reached room temperature.

Once thoroughly cooled and firm, use the parchment overhangs to lift the slab out of the pan. Transfer to a cutting board and slice into 12 generous bars or desired size. Wipe the knife clean between cuts for neat slices.

Optional: Dust with powdered sugar before serving. These bars are also delicious served warm (briefly reheated) with a scoop of vanilla ice cream or a dollop of fresh whipped cream.

Store leftover bars in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to 5-7 days. They can be frozen for up to 2-3 months; thaw at room temperature before serving.