For the topping, combine the flour, sugar and cinnamon in a small bowl. Cut in butter until mixture is crumbly; set aside.
For the batter, combine the flour, baking powder and salt; set aside. In a large bowl, cream the sugar, butter, egg and vanilla. Add flour mixture alternately with the milk, just until blended. Fold in blueberries. Pour into a greased and floured 9-in. square baking pan. Sprinkle topping over batter.
Bake at 375° for 40-45 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before gently removing from pan to wire rack to cool completely.