How to Make Buttermilk from Scratch: Scale this recipe up as needed. Place 2 teaspoons of vinegar or lemon juice in a liquid measuring cup. Fill cup with milk (2% or whole is best) until it reaches the 1/2-cup line. Let stand for five minutes. Use as directed.

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 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 m