1-First Step: Preheat and prepare Preheat your oven to 350°F and grease a 9×13-inch baking pan well. If you like a slightly crispier edge, line the pan with parchment and leave an overhang for easy lifting of the bars later.

2-Second Step: Make the crust and crumble In a food processor, combine 3 cups all-purpose flour, 1 1/2 cups sugar, and 1/4 teaspoon salt. Add 1 1/2 cups chilled butter (cut into cubes) and pulse until the mixture looks crumbly and sandy, with some pea-sized bits of butter left for flakiness. Reserve exactly 1 1/2 cups of this crumb mixture for the topping. Press the remaining crumb mixture firmly into the bottom of the prepared pan to form an even crust. Use the bottom of a measuring cup to press it down for a uniform layer. Bake the crust for 12 to 15 minutes until the edges are just golden brown. Remove the pan from the oven and let the crust cool for at least 10 minutes while you mix the filling.

3-Third Step: Whisk the filling In a medium bowl, whisk together 4 large eggs and 2 cups sugar until smooth and slightly glossy. Stir in 1 cup sour cream, 3/4 cup all-purpose flour, and a pinch of salt until just combined. The mixture should be pourable but thick enough to hold the berries.

4-Fourth Step: Add the blackberries Gently fold in 4 cups blackberries (fresh or defrosted and well-drained). If using frozen berries, drain them thoroughly in a colander and pat dry with paper towels before folding in to avoid a watery filling.

5-Fifth Step: Assemble and bake Spoon the blackberry filling evenly over the cooled crust, spreading gently with the back of a spoon to reach the corners without smashing the berries. Sprinkle the reserved 1 1/2 cups crumb mixture over the filling in an even layer. Bake at 350°F for 45 to 55 minutes, or until the top is lightly browned and the filling is set. Look for gentle bubbling in the center; that is a good sign the filling is cooked through.

6-Final Step: Cool and slice Remove the bars from the oven and cool for at least 1 hour on a wire rack. For clean slices, chill briefly after cooling, or use a sharp knife warmed under hot water and wiped dry between cuts. Cut into 24 bars and serve warm, at room temperature, or cold.