This is a classic!
For Cake (first 8 ingredients)
Preheat oven to 375 F.
Cream sugar and butter together. Mix egg and milk. Sift dry ingredients together and add to cream mixture, alternating with the liquid. Fold in blueberries. Pour batter into a greased 9 x 9 inch pan. Spread topping over cake batter and bake for 30 to 40 minutes.
For Topping (last 4 ingredients)
Sift sugar, flour, and cinnamon. Cut in butter until it forms lumps the size of peas. Spread on top of blueberry cake batter before baking.