Preheat the oven to 350 degrees. Cream butter with lemon zest and sugar (minus 1 tablespoon) 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 butter mixture a little at a time, alternating with the buttermilk; fold in the blueberries.
Grease a 9-inch square aluminum baking pan with butter or coat with non-stick spray. Spread batter into pan. Sprinkle batter with remaining tablespoon of sugar. Bake for 35-45 minutes. Check with a toothpick, checking for doneness. Let cool at least 15 minutes before serving.