blueberry crumb cake

the goods:

4 tablespoons unsalted butter, at room temperature, plus more for pan
1 and 1/2 cups all-purpose flour, plus more for pan and 1 teaspoon for blueberries
1 and 1/2 teaspoons baking powder
1/2 teaspoon baking soda
1/4 teaspoon salt
1/4 teaspoon ground allspice
3/4 cup sugar
1 large egg
1/4 cup buttermilk
1 and 1/2 cups blueberries, picked over, rinsed, and patted dry

streusel topping:
1 cup all-purpose flour
1/2 cup packed light brown sugar
1/4 teaspoon salt
1/2 cup (1 stick) cold unsalted butter, cut into tablespoons



Preheat oven to 350 degrees. Butter a 9-inch square baking pan; dust with flour, tapping out excess.



In a medium bowl, whisk together 1 and 1/2 cups flour, the baking powder, baking soda, salt, and allspice. In another bowl, cream the butter and sugar with an electric mixer until light and fluffy. Add the egg; beat until combined. Add the flour mixture and buttermilk in alternating batches, beginning and ending with the flour; beat until just combined.



In a small bowl, toss the blueberries with remaining teaspoon flour.



Gently fold the berries into the batter; pour into the prepared pan.



Make streusel: In a medium bowl, mix the flour, sugar, and salt. Using your fingers, work in the butter until large, moist crumbs form. Sprinkle streusel evenly on top of batter.



Bake until golden brown, 45 minutes. Let cool completely.