Preheat your oven to 375°F (190°C). Grease an 8x8 inch square baking pan and line it with parchment paper, leaving an overhang on two sides.

In a large bowl, whisk together the flour, 1/2 cup sugar, and salt. Cut in the cold butter with a pastry blender or your fingers until the mixture resembles coarse crumbs.

Reserve 1 1/2 cups of the crumble mixture and set it in the refrigerator. Press the remaining mixture firmly and evenly into the prepared pan to create a solid crust.

Bake the base for 12-15 minutes, or until the edges are very lightly brown. This pre-baking prevents a soggy bottom.

In a medium bowl, gently toss the fresh blackberries with the remaining 1/4 cup sugar, cornstarch, lemon zest, and lemon juice until well coated.

Remove the pre-baked base from the oven. Carefully spread the blackberry filling over the hot crust. Retrieve the reserved crumble from the refrigerator and sprinkle it evenly over the top.

Bake for 40-45 minutes, or until the filling is bubbling thickly, especially in the center, and the crumble topping is golden brown.

Allow to cool *completely* in the pan on a wire rack before lifting out by the parchment and slicing into wedges. Dust with powdered sugar if desired.