First Step: Prepare Oven and Pan
Preheat your oven to 350°F (175°C). Grease a 9 x 13-inch (23 x 33 cm) baking pan or line it with parchment paper for easy removal of the bars.

Second Step: Make the Crust Mixture
In a large bowl, combine 3 cups flour, 1 1/2 cups sugar, salt, and optional cinnamon, baking powder, and lemon zest if using. Cut in the chilled butter cubes with a pastry cutter or forks until the mixture reaches coarse crumbs. Reserve about 1 1/2 cups of this mixture for the topping.

Third Step: Form and Bake the Crust
Press the remaining crumb mixture firmly and evenly into the bottom of the prepared pan to create the crust. Bake for 12 to 15 minutes until the edges begin to brown. Let cool for about 10 minutes to set.

Fourth Step: Prepare the Filling
In a separate bowl, whisk the eggs and 2 cups sugar until combined. Stir in the sour cream or Greek yogurt, 3/4 cup flour, pinch of salt, and optional cornstarch, lemon juice, and lemon zest to stabilize and flavor the filling. Gently fold in the blackberries, being careful not to crush them.

Fifth Step: Assemble and Add Topping
Spread the blackberry filling evenly over the baked crust. Sprinkle the reserved crumb mixture on top loosely, avoiding pressing it down to keep the crumbly texture.

Final Step: Bake and Cool
Bake for 45 to 55 minutes or until the topping is lightly golden and the filling is set. Allow the bars to cool completely in the pan, preferably for at least an hour, before cutting into squares to let the filling firm up.