Preheat your oven to 350°F (175°C) and line an 8x8-inch baking dish with parchment paper.

Combine the all-purpose flour, rolled oats, light brown sugar, granulated sugar, baking powder, and salt in a large mixing bowl. Mix thoroughly with a whisk.

Incorporate the softened butter and vanilla extract until the mixture resembles large breadcrumbs.

Press half of the crumble mixture into the bottom of the lined baking dish to form a crust, then bake for 20 minutes.

While the crust bakes, prepare the blueberry filling by cooking fresh blueberries, granulated sugar, cornstarch, and lemon juice in a medium saucepan over medium heat for 2-3 minutes.

Remove the crust from the oven, spread the blueberry filling evenly over it, and sprinkle the remaining crumble mixture on top.

Return the pan to the oven and bake for an additional 30-35 minutes until the edges are golden brown and the blueberries are bubbling.

Let the bars cool completely in the pan for about 1-2 hours, then lift them out and slice into 16 squares.