Preheat the oven to 350 degrees. Grease a 9x9 or 7x11 baking dish.
In the bowl of a stand mixer, cream together the butter, lemon zest, and sugar until light and fluffy. Beat in the egg and vanilla until well combined.
Combine the flour, baking powder, and salt in a small bowl.
Add the flour mixture a little at a time to the butter mixture, alternating with the buttermilk.
Fold in the blueberries.
Spread the batter in the prepared baking dish and sprinkle with the 1 tablespoon of sugar.
Bake for 30 to 40 minutes, testing after 30 minutes.