Preheat the oven to 350 degrees. Coat an 8x8 baking dish with cooking spray.
Cream the butter, sugar, and lemon zest together with a mixer until creamy & smooth.
Add the egg and vanilla and beat until combined.
Add 2 tablespoons of flour to the blueberries and toss to coat evenly. Mix the remaining flour together with the baking powder and salt until well mixed.
Add the flour mixture to the butter mixture a little at a time alternating with the buttermilk. Fold in the blueberries gently.
Spread batter into the pan evenly. Sprinkle the 1 tablespoon of sugar evenly on top of the batter. Side note: The batter will be very thick
Place into the oven and bake for 45-50 minutes or until a tester inserted into the center of the cake comes out clean. Let it cool for a few minutes before cutting and serving. Enjoy!