For topping: ?½ cup sugar 1/4 cup all-purpose flour ?½ teaspoon cinnamon ?½ stick (1/4 cup) unsalted butter chilled and cut into bits Make cake batter: Onto a sheet of wax paper sift together flour, baking powder, and salt. In a bowl with an electric mixer cream butter and sugar until light and fluffy and beat in egg and milk. Add flour mixture in batches beating until just combined. Fold in blueberries. Preheat oven to 375¦F. Butter an 8-or 9- inch-square baking pan and dust with flour, knocking out excess. Make topping: In a bowl combine all topping ingredients until mixture resembles coarse meal. Spread batter into prepared pan and sprinkle evenly with topping. Bake cake in middle of oven 35 to 45 minutes, or until tester comes out clean.