Preheat oven to 350°. Butter an 8-inch square baking pan. Line bottom with parchment paper, leaving an overhang on two sides. Butter and flour paper, and tap out excess.
Whisk together melted butter, brown sugar, and 1/4 teaspoon salt. Add 1 cup of flour and mix with a fork until moist crumbs form. Refrigerate.
Whisk together 3/4 cup flour, baking powder, and 1/4 teaspoon salt. Set aside.
Using an electric mixer, beat softened butter, confectioners’ sugar, and vanilla until ligh and fluffy. Add eggs, one at a time, and beat well. Reduce mixer speed to low, and mix in flour mixture.
Spread batter evenly in prepared pan. Sprinkle with blackberries. Then, sprinkle with chilled topping. Bake 40-45 minutes, until golden. A toothpick inserted in the center should come out with moist crumbs attached. Cool completely in pan. Then, remove from pan using the overhanging parchment paper. Cut into squares.