Set out the butter and egg to come to room temperature. Spray an 8” square baking pan with cooking spray. Line the bottom of the pan with parchment paper. Preheat the oven to 350 degrees.
Remove 1 T. of sugar from the cup of sugar. Combine the remaining sugar with the butter and lemon zest. Beat with a mixer until the mixture is light and fluffy.
Add the egg and vanilla to the butter mixture and beat until combined.
Toss the blueberries with 1/4 c. of the flour. Set aside. Combine the remaining flour with the baking powder and salt.
Add half of the flour/baking powder mixture to the batter and stir until combined. Add all of the buttermilk and mix. Add the remaining flour and stir until combined. Fold in the blueberries. (Leave any excess flour from the blueberry bowl behind.)
Spread the batter into the prepared pan. Sprinkle the batter with the remaining 1 tablespoon of sugar.
Bake at 350 degrees for 35 to 40 minutes or until a toothpick comes out clean. Let cool for at least 15 minutes before serving.