If you don't have a 9-inch springform pan, you can bake this luscious breakfast cake in a 9x1-1/2-inch round baking pan. Just decrease blueberries to 1 cup.
Grease a 9-inch springform pan. Line the bottom with parchment or waxed paper; grease the paper. Flour the pan; set aside.
In a large mixing bowl stir together the 1 1/2 cups flour, the brown sugar, baking powder, baking soda, salt, and cinnamon. Using a pastry blender, cut in the 1/2 cup margarine or butter until mixture resembles coarse crumbs. Make a well in the center of the dry ingredients. In a small mixing bowl combine egg, buttermilk or sour milk, and vanilla. Add all at once to dry ingredients. Stir just until moistened.
Spread batter in prepared pan. Top with blueberries. In a small mixing bowl stir together the 1/4 cup all-purpose flour and granulated sugar. Cut in 2 tablespoons margarine or butter until mixture resembles coarse crumbs. Sprinkle over blueberries.
Bake in a 350 degree F. oven about 50 minutes or until a wooden toothpick inserted near center comes out clean.
Cool in pan on a wire rack for 30 minutes. Invert onto a plate and remove paper. Immediately invert the cake again onto a serving plate. Sprinkle with powdered sugar. Serve warm. Makes 10 servings.