Preheat the oven to 350º Fahrenheit. Spray an 8x8 baking pan with cooking spray.
In a large bowl, cream the butter and sugar together using a hand mixer, scraping the sides of the bowls as needed.
Add the egg and beat well.
In a small bowl, whisk together the flour, baking powder and salt.
Add the flour mixture alternately with the milk, mixing between each addition.
Fold in the blueberries.
Spread the batter in the prepared pan.
In a small bowl, use a fork to mix together the butter, sugar, flour and cinnamon until it resembles coarse crumbs.
Sprinkle the crumbs over the cake batter.
Bake for 33-35 minutes or until the cake tests done.
If you use a glass pan, bake at 325 F for about 40 minutes.The calories shown are based on the cake being cut into 12 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**