Preheat the oven to 350 degrees F (180 degrees C).
Using a stand mixer or hand-held mixer, cream the Unsalted Butter (1/2 cup) with the zest of the Lemon (1) and Granulated Sugar (1 cup) until light and fluffy.
Add the Egg (1) and Vanilla Extract (1 tablespoon) and beat until combined.
Meanwhile, toss the Fresh Blueberries (2 cup) with 1/4 cup of the All-Purpose Flour (2 cup). In another bowl, whisk together the remaining flour, Baking Powder (2 tablespoon) and Kosher Salt (1 teaspoon).
Add half of the flour mixture to the batter, and stir with spatula to incorporate. Add all of the Buttermilk (1/2 cup). Stir. Add remaining flour, and stir until flour is absorbed. Fold in the blueberries. Leave excess flour from the blueberry bowl behind.
Grease an 8 or 9-inch square baking pan with butter or coat with non-stick spray. If you have parchment paper on hand, line the pan with parchment on top of the butter. Spread the batter into the pan. Sprinkle the batter with the Granulated Sugar (1 tablespoon).
Bake for 35 to 45 minutes — a 9-inch pan will be done in closer to 35 minutes; an eight-inch pan usually needs 40 to 45 minutes. Check with a toothpick for doneness. If necessary, return pan to oven for a couple of more minutes.
Let cool at least 15 minutes before serving.