- 6 tablespoons unsalted butter melted, and 1/2 cup (1 stick), room temperature, plus more for pan
- 1 3/4 cups all-purpose flour (spooned and leveled), plus more for pan
- 1/2 cup packed light-brown sugar
- 1/2 teaspoon salt
- 1/2 teaspoon baking powder
- 1 cup confectioners' sugar
- 1/2 teaspoon pure vanilla extract
- 2 large eggs
- 2 containers (5 ounces each) blackberries
- Preheat oven to 350 degrees. Butter an 8-inch square baking pan. Line bottom with parchment paper, leaving an overhang on two sides; butter and flour paper, tapping out excess.
- Make topping: In a medium bowl, whisk together melted butter, brown sugar, and 1/4 teaspoon salt; add 1 cup flour, and mix with a fork until large moist crumbs form. Refrigerate topping until ready to use.
- In a medium bowl, whisk together remaining 3/4 cup flour, baking powder, and remaining 1/4 teaspoon salt; set aside. In a large bowl, using an electric mixer, beat room-temperature butter, confectioners' sugar, and vanilla until light and fluffy; add eggs, one at a time, beating well after each addition. Reduce speed to low; mix in flour mixture. Spread batter evenly in pan; sprinkle with blackberries, then chilled topping.
- Bake until golden and a toothpick inserted in center comes out with moist crumbs attached, 40 to 45 minutes. Cool completely in pan. Using paper overhang, lift cake onto a work surface; cut into 16 squares.
While you prepare the cake, refrigerate the crumb topping. This will help give it a nubbly texture once baked. To store, keep in an airtight container at room temperature, up to 3 days.