½ cup unsalted butter, room temperature
2 tsp. lemon zest or more — zest from 1 large lemon
⅞ cup* + 1 tablespoon sugar for sprinkling
1 egg, room temperature
1 tsp. vanilla
2 cups flour (set aside ¼ cup of this to toss with the blueberries)
2 tsp. baking powder
1 tsp. kosher salt
2 cups fresh blueberries
½ cup buttermilk**
* ⅞ cup = ¾ cup + 2 tablespoons
** To make homemade buttermilk, place 1 tablespoon of vinegar or lemon juice in a liquid measuring cup. Fill cup with milk until it reaches the 1-cup line. Let stand for five minutes. Use only ½ cup of the prepared mixture for the recipe.
Preheat the oven to 350ºF. Cream butter with lemon zest and ⅞ cup of the sugar until light and fluffy.
Add the egg and vanilla and beat until combined. Meanwhile, toss the blueberries with ¼ cup of flour, then 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 (or something similar) with butter or coat with non-stick spray. Spread batter into pan. Sprinkle batter with remaining tablespoon of sugar. Bake for 35 to 45 minutes. Check with a toothpick for doneness. If necessary, return pan to oven for a couple of more minutes. (Note: Baking for as long as 10 minutes more might be necessary.) Let cool at least 15 minutes before serving.