1-Preheat: Preheat your oven to 375 degrees Fahrenheit and butter a 9×13-inch baking dish, optionally lining it with parchment paper for easy removal these steps set the stage for success.

2-Combine dry ingredients: First, in a large bowl, whisk together the 3 cups all-purpose flour, 1/2 cup granulated sugar, 1/2 cup brown sugar, 1 teaspoon baking powder, and 1/4 teaspoon salt until everything blends smoothly.

3-Mix wet ingredients and combine: In a separate bowl, whisk the 1 egg and 1 1/2 teaspoons vanilla extract until combined, then add the 1 cup chilled butter cubes to the flour mixture along with this egg mix. Use a fork or pastry cutter to work it into coarse crumbs, which gives the bars their signature texture.

4-Form base layer: Next, press half of the crumb mixture evenly into your prepared baking dish to form a solid base layer.

5-Prepare blueberry filling: In another bowl, whisk the remaining 1/2 cup granulated sugar with 1 tablespoon cornstarch, then stir in 1 1/2 tablespoons lemon juice. Add the 4 cups fresh blueberries and toss them gently to coat, ensuring the filling holds together without becoming too runny.

6-Assemble and bake: Pour the blueberry mixture over the crust, then sprinkle the rest of the crumb mixture on top to cover it completely. Bake for 38 to 42 minutes until the top turns golden brown, and let the bars cool for 20 to 30 minutes before cutting them into squares. This cooling time helps the filling set, making it easier to store in an airtight container in the fridge for later.