How to Make It
Preheat oven to 350°.
Coat a 9-inch round cake pan with cooking spray; line bottom with wax paper. Coat wax paper with cooking spray.
Beat 2/3 cup sugar and 1/4 cup margarine at medium speed of a mixer until blended (about 5 minutes). Add vanilla and egg; beat well. Lightly spoon 1 1/3 cups flour into dry measuring cups; level with a knife. Combine 1 1/3 cups flour, baking powder, baking soda, and salt. Add flour mixture to creamed mixture alternately with buttermilk, beginning and ending with flour mixture. Stir in blueberries.
Pour into prepared pan. Combine 3 tablespoons sugar and remaining ingredients in a bowl; stir until mixture resembles coarse meal. Sprinkle sugar mixture over batter. Bake at 350° for 40 minutes or until a wooden pick inserted in center comes out clean. Cool on a wire rack.