Preheat the oven to 350°F and line an 8-inch square baking pan with parchment, leaving a 1–2 inch overhang for easy removal.

In a bowl, cream 1 cup cold unsalted butter cubes and 3/4 cup light brown sugar until smooth, about 1–2 minutes; add 1 large egg yolk and 1 teaspoon vanilla and mix until combined.

Whisk together 2 cups all-purpose flour, 1/2 teaspoon kosher salt, and 1/2 teaspoon baking powder, then add to the butter mixture and stir until a soft dough forms.

Press the dough evenly into the prepared pan and bake for 18 minutes, until edges are lightly golden and center is set.

Combine 3 cups blackberries with 1/3 cup granulated sugar and 1 tablespoon lemon juice and let sit 10 minutes; mix 1 tablespoon cornstarch with 1 tablespoon water and stir into the berries.

If using frozen blackberries, thaw and drain excess liquid before macerating.

Remove the pre-baked shortbread from the oven, spread the blackberry mixture evenly on top, and return to the oven for 14–16 minutes until the fruit bubbles at the edges and the topping looks set.

Cool the pan on a wire rack for 20 minutes, then chill in the refrigerator for 20–30 minutes for easier slicing.

Lift the slab using the parchment overhang, place on a cutting board, and cut into 8 wedges. For clean cuts, warm and dry a knife between slices.

If desired, drizzle a simple lemon glaze (powdered sugar mixed with lemon juice) over cooled wedges or sprinkle chopped toasted nuts before the final bake for crunch.