Preheat the oven to 350 degrees F. Cream butter with lemon zest (if you opt to use it) and 3/4 cups of sugar until light and fluffy.
Add the egg and vanilla and beat until combined. Meanwhile, toss the blueberries with 1/4 cup of flour, then whisk together the remaining flour, baking powder and salt.
Add the flour mixture to the batter a little at a time, alternating with the buttermilk. Fold in the blueberries. If you use frozen berries, allow them to thaw and drain out the excess liquid.
Grease a 9-inch square baking pan (or something similar) with butter or coat with non-stick spray. Spread batter into pan. Sprinkle batter with an extra tablespoon of sugar. Bake for 35 minutes. Check with a toothpick for doneness. If necessary, return pan to oven for a couple of more minutes. (Note: Baking for as long as 10-minutes more might be necessary.) Let cool at least 15 minutes before serving.