Preheat the oven to 350 degrees F. Cream butter, lemon zest and 3/4 cup + 1 Tablespoon of the granulated sugar together. (Set aside the remaining Tablespoon of sugar for later.) Beat the mixture until creamy and fluffy. Add egg and vanilla and beat until just combined.
Meanwhile, toss the blueberries with 1/4 cup of the flour. Set aside.
Whisk together the remaining flour, baking powder, and salt together in a separate bowl. Slowly add flour mixture to wet mixture, alternating with the buttermilk. Gently fold in blueberries.
Grease a 9×9-inch square baking dish (or an 8x10-inch baking dish) with non-stick spray. Spread batter evenly into the prepared pan (batter will be thick). Sprinkle the remaining tablespoon of sugar evenly over the top of the batter.
Bake for 35 minutes or until a toothpick inserted comes out clean. Let cool for 15 minutes before slicing. Serve and enjoy!