For a fruity variation of this classic crumb cake, stir blueberries into the batter, and sprinkle over the top along with the crumb topping.
Preheat oven to 350 degrees. Sift together flour, baking soda, baking powder, and salt into a bowl; set aside. Butter a 9-inch square baking pan; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream butter and granulated sugar until light and fluffy, about 4 minutes. Add eggs, one at a time, until well combined. Add vanilla, and stir until combined. Add reserved flour mixture and the sour cream, and stir just until well combined. Fold in 2 cups blueberries, if using. Spoon batter into prepared pan. Toss remaining cup blueberries, if using, with the crumb topping. Sprinkle the crumb topping over cake. Bake until golden brown and cake tester comes out clean, 50 to 60 minutes. Dust with confectioners' sugar before serving, if desired.