Preheat oven to 375 degrees F. Grease a 9x9 baking pan and set aside.
In a large mixing bowl, with a mixer, cream together butter and sugar until light and fluffy. Add in egg and mix until well combined. Add in vanilla and mix until well combined.
In a separate bowl combine flour, baking powder and salt, mix well. Add half the dry ingredients to the wet ingredients and mix until just combined. Add in half the buttermilk and mix until just combined. Add remaining dry ingredients and mixed until just combined. Add in remaining buttermilk and mix until combined. Gently fold in blueberries. Scoop into prepared baking pan.
For topping, mix together brown sugar, flour and cinnamon until well combined. Cut in cold butter and mix until a crumb like mixture is created. Sprinkle over cake batter.
Bake for approximately 30-35 minutes or until a toothpick inserted into the center comes out clean.
*thawed, frozen blueberries can be used, but it will increase the baking time