Preheat the oven to 350F degrees. Line a regular sized muffin tin with muffin/cupcake papers.
In a large bowl using an lectric mixer on medium speed, beat together the butter and sugar until light and fluffy. Add in the egg, egg white and vanilla and continue beating until combined. Turn the mixer down to medium-low speed and add in about half of the flour mixture, followed by about half of the buttermilk. Add in the rest of the flour mixture and beat on low speed until almost combined, then add in the rest of the buttermilk and beat on low until all mixed in. Be careful not to over mix the batter.
Decorate the cupcakes using a knife, or a piping bag and your favorite piping tip (I typically use a star tip, such as the Wilton 1M).
*To make your own cake flour, measure 1 and 1/3 cups all-purpose flour. Remove 2 tablespoons and 2 teaspoons of the all-purpose flour. Then add in 2 tablespoons and 2 teaspoons of cornstarch. Whisk together.
**If you don't have buttermilk, whole milk or 2% milk can be substituted. I don't recommend skim milk however, as it doesn't have as much flavor.