1. Preheat oven to 375 degrees.
2. In a large bowl cream butter and sugar together with hand mixer. Add in your egg and beat. Add flour, baking powder, salt and continue to mix. Add vanilla and milk and mix one more time.
3. Add in blueberries and mix around with a spoon so blueberries are equally distributed.
4. Spray a 8x8 pan with nonstick spray. Pour your cake batter into the pan.
5. In a small bowl combine sugar, flour and cinnamon. Add in your softened (not melted!) butter and mix with a fork to make a crumble mixture. Once crumbly, sprinkle on top of your cake.
6. Bake for 45 to 47 minutes. Let cool for at least 15 minutes. Cut cake into preferred size pieces (I usually cut it into 18 pieces).