1. Preheat your oven to 375 F (190 C). Lightly grease a 9×13-inch baking pan, or line it with parchment paper, leaving an overhang on the sides.

2. In a large mixing bowl, combine the softened butter, 1 cup of granulated sugar, and vanilla extract. Beat with an electric mixer on medium speed until light and fluffy, about 2-3 minutes. In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined and a crumbly mixture forms. It should look like coarse crumbs with some larger clumps.

3. Press about two-thirds of the crumb mixture evenly into the bottom of your prepared baking pan. Use the back of a spoon or your clean hands to create a firm, compact base.

4. In a medium bowl, gently combine the blueberries (fresh or frozen), the remaining 1/2 cup of granulated sugar, cornstarch, and lemon juice (if using). Stir until the blueberries are evenly coated.

5. Pour the blueberry mixture evenly over the pressed crust in the baking pan. Spread it gently to cover the entire surface. Then, sprinkle the remaining one-third of the crumb mixture over the blueberry filling. Do not press this top layer down; you want it to remain crumbly for that classic crumble texture.

6. Place the baking pan in the preheated oven and bake for 45-50 minutes, or until the crumble topping is golden brown and the blueberry filling is bubbly around the edges.

7. Once baked, remove the pan from the oven and place it on a wire rack to cool completely. Allow the bars to cool for at least 30 minutes, or preferably longer, before attempting to slice them.

8. Once fully cooled, use the parchment paper overhang to lift the entire slab of bars out of the pan. Place it on a cutting board and slice into squares or rectangles of your desired size. Serve and enjoy!