Combine flour, baking powder, and a salt. Stir and set aside.
Cream 1/2 stick plus 1 tablespoon butter with cinnamon and sugar. Add egg and mix until combined. Add vanilla and mix. Add flour mixture and milk alternately until totally incorporated. Do not overbeat. Stir in blueberries until evenly distributed.
Grease a 9 x 13 inch baking pan. Pour in batter.
In a separate bowl, combine topping ingredients and cut together using two knives or a pastry cutter. Sprinkle over the top of the cake.
Bake cake for 40 to 45 minutes, or until golden brown. Sprinkle with sugar. Cut into squares and serve with softened butter.