1. Preheat your oven to 375 F (190 C). Lightly grease a 9×13-inch baking pan, then line it with parchment paper, leaving an overhang on the sides. This parchment “sling” will make it much easier to lift the cooled bars out of the pan for cutting.

2. In a large mixing bowl, combine the softened butter, 3/4 cup of the 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 dough forms. The mixture should resemble coarse crumbs.

3. Press about two-thirds of the crumb mixture evenly into the bottom of the prepared 9×13-inch baking pan. Use your hands or the bottom of a flat glass to compact it firmly. This forms the buttery crust for your bars.

4. In a medium bowl, gently toss the fresh blueberries with the remaining 1/4 cup of granulated sugar, cornstarch, and lemon juice. Ensure the blueberries are evenly coated. The cornstarch will help thicken the juices as they bake, creating a luscious filling.

5. Pour the blueberry mixture evenly over the pressed crust in the baking pan. Spread it gently to cover the entire surface. Then, crumble the remaining one-third of the crumb mixture evenly over the blueberry filling. This will form your signature crumbly topping.

6. Place the baking pan in the preheated oven and bake for 40-45 minutes, or until the topping is golden brown and the blueberry filling is bubbly and set. You’ll notice the edges turning a deeper golden color and the fruit juices thickening.

7. Remove the pan from the oven and place it on a wire rack to cool completely. This is a critical step! Allow the bars to cool for at least 1 hour, or even longer, before lifting them out of the pan using the parchment paper overhang. Once lifted, transfer them to a cutting board and slice into squares or bars.