Preheat oven to 350 degrees

Whisk together flour, sugar, baking soda and baking powder in a large mixing bowl.

Add butter, egg whites, vanilla, sour cream and milk and mix on medium speed just until smooth. Do not over mix.

Fill cupcake liners a little more than half way.

Bake 18-20 minutes.

Allow to cool for 1-2 minutes, then remove to cooling rack to finish cooling.

Icing:

Combine butter and shortening and mix until smooth.

Add 2 cups of powdered sugar and mix until smooth.

Add vanilla extract and 1-2 tbsp of water or cream and mix until smooth.

Add remaining powdered sugar and mix until smooth.

Add a little more water or cream until desired consistency is reached.

Top cupcakes with icing.

Note: You may notice little bits of butter in the batter, but that's ok. They will melt into the cupcake as they bake