Preheat the oven to 350ºF
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 2 tablespoons of flour, then whisk together the remaining flour, baking powder and salt.
Add half of the flour mixture to the batter, mix to incorporate. Add all of the buttermilk. Stir until combined. Add remaining flour, and stir until combined. Fold in the berries.
Grease a 9x13 baking dish with butter or non-stick spray. Sprinkle the batter with the remaining tablespoon of sugar. Bake for 30-35 minutes. Check with a toothpick for doneness. Let cool at least 15 minutes before serving. Can be served cold or warm. Store, covered, on the counter for up to 4 days.