In another large bowl, whisk together the egg, mashed banana, water, milk, oil, and vanilla extract. Add the wet ingredients to the dry ingredients and stir, or whisk, until combined. (The batter is quite thin.) Pour or scoop the batter into the muffin cups, about 3/4 full, and bake for about 20 minutes or until a toothpick inserted in the center of a cupcake comes out clean.