In a small bowl, cream butter and sugar until light and fluffy. Beat in egg. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk, beating well after each addition. Fold in blueberries. Pour into greased 9-in. square baking pan.
For topping, in a small bowl, combine the sugar, flour and cinnamon; cut in butter until crumbly. Sprinkle over blueberry mixture.
Bake at 375° for 40-45 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack. Yield: 4-6 servings.