Buttermilk-Blueberry Breakfast Cake

1/2 cup unsalted butter, room temperature
2 teaspoons lemon zest
7/8 cup + 1 tablespoon sugar *this tablespoon is for sprinkling on top
1 egg, room temperature
1 teaspoon vanilla
2 cups flour
2 teaspoons baking powder
1 teaspoon kosher salt
2 cups fresh blueberries
1/2 cup buttermilk

Preheat oven to 350 degrees.  Cream butter, lemon zest, 7/8 cup sugar until light and fluffy.  Add the egg, vanilla and beat until combined.  Meanwhile, toss the blueberries with 1/4 cup of the flour.  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 butter or coat with non-stick spray.  Spread batter into pan and sprinkle with remaining tablespoon of sugar.  Bake for 35 minutes.  Check with a toothpick for doneness.  Cool for 15 minutes before serving.  Good enough for dessert if topped with whipped cream or ice cream.