Preheat the oven to 350ºF.
Cream butter and sugar until light and fluffy.
Add the egg and vanilla and beat until combined.
Toss the blueberries with ¼ cup of flour.
In a separate bowl, 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.
Grease a 9-inch square baking pan with non-stick spray. Spread batter into pan.
Sprinkle batter with remaining tablespoon of sugar.
Bake for 35-45 minutes. Check with a toothpick for doneness.
Let cool at least 15 minutes before serving.