In a large bowl, beat butter at medium speed of an electric mixer until creamy; gradually add sugar, beating well. Add egg, beating well.
Combine flour, baking powder, and salt in a small bowl; add to butter mixture alternately with milk, beginning and ending with flour mixture. Mix at low speed after each addition until blended. Stir in vanilla. Pour batter into a 9-inch round cake pan coated with cooking spray. Sprinkle blueberries over top of batter. Sprinkle Streusel Topping over blueberries.
Bake at 350° for 45 to 47 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack. Cut into wedges to serve.