Preheat the oven to 350 degrees F. Grease a 9-inch square baking pan with butter or coat with non-stick spray and set aside.
Cream butter with lemon zest and 3/4 cup plus 2 tbsp. of the sugar until light and fluffy.
Add the egg and vanilla and beat until combined.
Meanwhile, toss the blueberries with 1/4 cup of flour.
In a separate bowl, whisk together the remaining 1 3/4 cup 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.
Spread batter into the prepared pan. Sprinkle batter with remaining 1 tbsp. of sugar. Bake for 35 to 45 minutes. Check with a toothpick for doneness. If still wet, return pan to oven for a couple of more minutes. (Note: Baking for as long as 10 minutes more might be necessary.)