- For the Streusel Topping:
- 1 cup all-purpose flour (spooned and leveled)
- 1/2 cup packed light-brown sugar
- 1/4 teaspoon salt
- 1/2 cup butter
- For the Cake:
- 4 tablespoons unsalted butter, softened, plus more for pan
- 1 1/2 cups all-purpose flour, plus more for pan
- 1 1/2 teaspoons baking powder
- 1/2 teaspoon baking soda
- 1/4 teaspoon salt
- 1/4 teaspoon allspice
- 3/4 cup sugar
- 1 large egg
- 2/3 cup buttermilk
- 1 1/2 cups blueberries
- 1 teaspoon flour
- Confectioners' sugar, for dusting
- Preheat oven to 350 degrees F. Butter and flour a 9-inch square baking pan. Make the topping: In a medium bowl, stir together flour, brown sugar, and salt. Cut in the butter using your hands or a pastry blender until large, moist crumbs form. Chill.
- In a medium bowl, whisk together 1 1/2 cups flour, baking powder, baking soda, salt, and allspice. In a large bowl, beat the butter and sugar with an electric mixer until fluffy. Add egg; beat well. Add flour mixture and buttermilk alternately until just combined. (Batter will be very stiff.) In a large bowl, toss the blueberries with 1 teaspoon flour. Fold blueberries into the batter; spoon into prepared pan.
- Sprinkle cake with streusel topping. Bake until golden brown and a tester comes out clean, 45 to 50 minutes. Let cool completely. Dust with confectioners' sugar before cutting into squares.