For the cake: Preheat the oven to 350 degrees F. Grease a 9-by-13-inch baking pan with butter.
Add the flour, baking powder, cinnamon and salt to a medium bowl and stir to combine. Set aside.
Cream together the butter and sugar with an electric mixer. Add the egg and mix until combined. Mix in the vanilla. Add the flour mixture and milk alternately and mix until totally incorporated. Do not over-beat. Stir in the blueberries until evenly distributed. Pour the batter into the prepared pan.
For the topping: Combine the flour, sugar, butter, cinnamon and salt in a medium bowl and cut together using 2 knives or a pastry cutter. Sprinkle over the top of the batter.
Bake until the cake is golden brown, 40 to 45 minutes. Sprinkle with sugar. Cut into squares and serve with softened butter.