½ cup (8 tablespoons | 4 oz | 113 g ) unsalted butter, room temperature
zest from 1 large lemon
1 cup (214 g) sugar (set aside 1 tablespoon for sprinkling)
1 egg, room temperature
1 tsp. vanilla
2 cups (256 g) all-purpose flour (set aside 1/4 cup of this to toss with the blueberries)
2 tsp. baking powder
1 tsp. kosher salt (I like 1.25 tsp)
2 cups fresh blueberries, picked over
½ cup buttermilk, see FAQs below
Preheat the oven to 350ºF. Using a stand mixer or hand-held mixer, cream the butter with the lemon zest and the 1 cup minus 1 tablespoon of 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 half of the flour mixture to the batter, and stir with spatula to incorporate. Add all of the buttermilk. 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 (or something similar—I prefer this 8-inch pan because I like the thicker pieces) 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 remaining tablespoon of sugar. Bake for 35 to 45 minutes — a 9-inch pan will be done in closer to 35 minutes; an 8-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. (Note: Baking for as long as 10 minutes more might be necessary, especially if you’re using a smaller pan such as an 8×8-inch. It’s not unusual for this cake to take 50 minutes, so just be patient.) Let cool at least 15 minutes before serving.