Preheat the oven to 180ºC. Using a stand mixer or hand-held mixer, cream the butter with the lemon zest and the 1 cup minus 1 tablespoon of sugar until light and fluffy.
Add the egg and vanilla and beat until combined. Meanwhile, toss the blueberries with ¼ cup of flour, then whisk together the remaining flour, baking powder and salt.
Add half of the flour mixture to the batter, and stir with spatula to incorporate. Add all of the buttermilk. Stir. Add remaining flour, and stir until flour is absorbed. Fold in the frozen blueberries.
Grease a baking pan with butter or coat with non-stick spray. Spread the batter into the pan. Sprinkle the batter with the remaining tablespoon of sugar. Bake for 35 to 45 minutes. Check with a toothpick. If necessary, return pan to oven for a couple of more minutes
Let cool at least 15 minutes before serving. Enjoy!