Preheat the oven to 350 degrees. Grease a 9-inch square baking dish with butter.

In a small bowl, toss the blueberries with ¼ cup of flour; set aside. Whisk together the remaining flour, baking powder and salt in a medium bowl and set aside.

In the bowl of an electric mixer fitted with the paddle attachment, cream the butter, lemon zest and ¾ cup plus one tablespoon of the sugar together until light and fluffy. Add the egg and vanilla and beat until combined.

Add the flour mixture to the batter a little at a time, alternating with the buttermilk. Fold in the blueberries.

Spread batter into greased baking dish and sprinkle batter with remaining tablespoon of sugar. Bake for 35-45 minutes, or until top is slightly golden brown and a toothpick inserted into the center comes out clean. Allow to cool for 10-15 minutes before serving.