1. Preheat the oven to 350°F (176°C). Line three 8-inch cake pans with parchment paper in the bottom and grease the sides.
2. Beat the butter and sugar in a large bowl on medium speed about 2 minutes, until light and fluffy. Scrape down the sides of the bowl.
3. Add the egg whites and vanilla and beat on medium speed for about 1 minute, no more. The texture of the batter will change and become thicker and may seem a little curdled.
4. Combine the flour, baking soda and baking powder in a medium sized bowl.
5. Add about one-third of the flour mixture to the batter and beat on medium speed until incorporated.
6. Add about half of the milk and beat on medium speed until incorporated.
7. Continue alternating adding dry and wet ingredients, scraping down the sides of the bowl as needed, and beating until incorporated after each addition. There should be three total additions of dry ingredients and two total additions of milk. End with the dry ingredients. The batter will be thick and glossy.
8. Divide the batter evenly between the cake pans and spread evenly.
9. Stagger the cake layers on the oven racks so that no layer is directly over another. Bake for 30-33 minutes, or until a toothpick inserted into the middle of the cakes comes out clean. Don’t be alarmed if the tops are a little browned, that is normal.
10. Remove cakes from oven and allow to cool for 3-5 minutes.
11. Remove cakes from pans to cooling racks and cool completely before frosting.
12. To make icing, combine butter and shortening and mix until smooth.
13. Add 6 cups of powdered sugar and mix until smooth.
14. Add vanilla extract and 4 tbsp of water or milk and mix until smooth.
15. Add remaining powdered sugar and mix until smooth.
16. Add more water or milk until desired consistency is reached.
17. To assemble cake, place first layer of cake on your cake stand or plate. NOTE: The cake layers shouldn’t have a rounded dome on top of them, but if they do, cut them off before stacking.
18. Add about 1 1/2 cups icing and spread into an even layer.
19. Add second layer of cake and another 1 1/2 cups of icing.
20. Top cake with remaining layer of cake.
21. Use the rest of the icing to ice the outside of the cake. For instructions on icing a smooth cake and the piping, here is a tutorial on How to Frost a Smooth Cake.

TO MAKE CUPCAKES:

Follow instructions above through number 7. Fill cupcake liners about 1/2 way. Bake at 350 degrees for about 16-18 minutes. Allow to cool in pan for 2-3 minutes, then remove to cooling rack to cool completely. Recipe makes about 36 cupcakes.