Slowly, add in water, sour cream, egg whites, vegetable oil, and vanilla extract. Mix until everything has been combined.
Pour or spoon batter into the lined cupcake pan. Make sure to only fill about ⅔ of the cupcake liner.
Cut out the center of the cupcakes and fill with 1- 2 teaspoons of sprinkled candy mixture. Add top back to the cupcake to hold filling in.
Preparing Frosting- With an electric hand mixer or a kitchen aid, cream together butter and shortening until smooth and creamy, about 3 minutes.