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.