Step 1: Preheat your oven to 375°F (190°C). Grease and flour a 9×13 inch baking pan, or line it with parchment paper, leaving an overhang on the sides for easy lifting.
Step 2: In a large bowl, whisk together 2 ¼ cups all-purpose flour, 1 cup granulated sugar, 1 teaspoon baking powder, and ¼ teaspoon salt.
Step 3: Add the 1 cup cold unsalted butter (cut into pieces) to the dry ingredients. Using a pastry blender, your fingertips, or a food processor, cut the butter into the flour mixture until coarse crumbs form. Don’t overmix; you want small pea-sized pieces.
Step 4: Press about two-thirds of the crumb mixture evenly into the bottom of the prepared baking pan. Press firmly to create a compact crust.
Step 5: In a separate bowl, combine 4 cups hulled and sliced fresh strawberries, ½ cup granulated sugar, 2 tablespoons all-purpose flour, and 1 teaspoon vanilla extract. Gently toss until the strawberries are evenly coated.
Step 6: Spread the strawberry filling evenly over the crust in the baking pan.
Step 7: Crumble the remaining one-third of the crumb mixture evenly over the strawberry layer.
Step 8: Bake for 45-55 minutes, or until the topping is golden brown and the strawberry filling is bubbly.
Step 9: Let the bars cool completely in the pan on a wire rack before cutting. This is essential for clean cuts and for the filling to set properly.