Preheat oven to 350°F. Line a cupcake tin with cupcake liners. In a large bowl, whisk together dry ingredients. In another bowl, whisk egg, water, buttermilk, vegetable oil and vanilla. Stir wet ingredients into dry ingredients; mix batter until smooth, about 3 minutes.
Fill each cup about 2/3 of the way. Bake, rotating pan halfway through baking, until a toothpick inserted in center comes out clean, 20 to 25 minutes for regular cupcakes and about 10 minutes for mini cupcakes. Let cool on wire rack.