1-First Step: Prepare Your Baking Setup Start by preheating your oven to 350 degrees F (175 degrees C). Proper oven temperature is crucial for even baking. While the oven heats, grease a 9 x 13 baking pan thoroughly. You can use butter, cooking spray, or line the pan with parchment paper for easier removal. If using parchment, leave some overhang on the sides this makes lifting the bars out much simpler after cooling.

2-Second Step: Create the Crust and Crumb Mixture This step is where a food processor becomes your best friend. Combine the 3 cups all purpose flour, 1 1/2 cups sugar, and 1/4 teaspoon salt in your food processor. Pulse a few times to blend everything evenly. Next, cut your 1 1/2 cups chilled butter into 1/2 inch cubes. Cold butter is essential here warm butter will make the crust tough instead of flaky. Add the butter cubes to the flour mixture in the food processor. Process until the mixture becomes crumbly and starts to clump together when pressed. This usually takes about 30-45 seconds of processing. Measure out 1 1/2 cups of the crumb mixture and set it aside for the topping. This reserved portion will create that beautiful streusel-like finish on your bars.

3-Third Step: Press and Par-Bake the Crust Take the remaining crumb mixture and press it firmly into the bottom of your prepared 9 x 13 pan. Use the bottom of a measuring cup or your hands to create an even, compact layer. Work from the center outward to ensure uniform thickness. Place the pan in your preheated oven and bake the crust for 12 to 15 minutes. You’re looking for the edges to begin browning slightly. This par-baking step ensures your crust won’t become soggy once the filling is added. After removing from the oven, let the crust cool for at least 10 minutes. This cooling period is important adding the filling to a hot crust can cause the eggs to scramble, creating an undesirable texture.

4-Fourth Step: Prepare the Blackberry Filling While the crust cools, prepare your filling. In a large bowl, whisk 4 large eggs and 2 cups sugar together until well combined and slightly frothy. This takes about 1 minute of vigorous whisking. Add the 1 cup sour cream (or Greek yogurt), 3/4 cup flour, and pinch of salt to the egg mixture. Whisk until smooth with no lumps remaining. The filling should have a creamy, pourable consistency. Gently fold in the 4 cups blackberries. If using frozen berries, make sure they’re completely thawed and well-drained. Excess moisture will make your bars watery. Use a spatula to fold the berries in carefully you want them coated with the filling mixture but not crushed.

5-Fifth Step: Assemble and Bake Pour the blackberry filling over your cooled crust, spreading it evenly to all corners. A spatula helps distribute the berries uniformly throughout the pan. Sprinkle the reserved 1 1/2 cups of crumb mixture evenly over the top. The crumbs should cover most of the filling but don’t worry about complete coverage some spots of filling showing through is normal and creates visual interest. Return the pan to the oven and bake for 45 to 55 minutes. The bars are done when the crust is lightly browned and the filling appears set with only a slight jiggle in the center. A toothpick inserted near the center should come out mostly clean with a few moist crumbs.

6-Sixth Step: Cool and Serve Patience is key here. Cool the bars for at least 1 hour before cutting. This cooling time allows the filling to set properly, making clean cuts possible. Cutting too soon will result in messy, fallen-apart bars. For the neatest slices, refrigerate the cooled bars for another hour before cutting. Use a sharp knife dipped in hot water and wiped clean between cuts.