Preheat oven to 350°F (175°C). Butter an 8-inch square baking pan. Line the bottom with parchment paper, leaving an overhang on two sides. Butter and flour the paper, and tap out the excess.
Whisk together the melted butter, brown sugar, and 1/4 teaspoon salt. Add 1 cup (120g) of the flour and mix with a fork or a pastry blender until moist crumbs form. Refrigerate.
Whisk together the remaining 3/4 cup (90g) flour, baking powder, and 1/4 teaspoon salt. Set aside.
Using an electric mixer on medium speed, beat the butter, confectioners’ sugar, and vanilla until light and fluffy. Add the eggs, one at a time, mixing well after each addition.
Reduce mixer speed to low, and mix in the flour mixture just until combined.
Spread the batter evenly in the prepared pan. Sprinkle with blackberries. Then, sprinkle with the chilled topping.
Bake 40 to 45 minutes, or until golden. A toothpick inserted in the center should come out with moist crumbs attached.
Cool completely in the pan on a wire rack. Then, remove from the pan using the overhanging parchment paper and cut into bars.
Notes
Makes 16 standard servings or 12 generous servings.
Store in an airtight container at room temperature up to 3 days.