Preheat oven to 350°F and line pan with cupcake liners.
Combine dry cake mix, butter, sour cream, milk, extract, and egg whites. Blend at low speed until moistened (about 30 seconds) and then medium speed for 2 minutes.
Fill each cupcake liner 2/3 full.
Bake for 18-21 minutes or until a toothpick inserted in the center comes out clean.
Allow cupcakes to cool.
Use a small knife to cut out the core of the cupcake and fill with marshmallow cream.
In an electric mixer, combine butter and remaining marshmallow cream (7 ounces) and cream together for 2 minutes.
Add powdered sugar 1 cup at a time. Add milk. Beat for 2 minutes.
If you want a thicker consistency, add more powdered sugar. For a looser consistency, add more milk. Keep in mind, a thicker consistency will be easier to pipe.
Pipe frosting onto cupcakes and top with a marshmallow heart!