1. Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
2. Beat butter and sugar until light in color and fluffy, about 2-3 minutes.
3. Add sour cream and vanilla extract and mix until well combined.
4. Add egg whites in two batches, mixing until well combined after each. Scrape down the sides of the bowl to make sure all is incorporated.
5. Combine dry ingredients in another bowl, then combine milk and water in another bowl.
6. Add half of the dry ingredients to the batter and mix until well combined. Add the milk mixture and mix until well combined. Add remaining dry ingredients and mix until well combined.
7. Fill cupcake liners about halfway. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs.
8. Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to cooling rack to finish cooling.
9. To make the frosting, beat the butter and shortening together until smooth.
10. Add half of the powdered sugar and beat until smooth.
11. Add vanilla extract and 2 tbsp of water or milk and beat until smooth.
12. Add Oreo crumbs and mix until combined.
13. Add remaining powdered sugar and beat until smooth.
14. Add another 2-3 tablespoons of water or milk, as needed, and beating until smooth and a nice consistency for piping.
15. To decorate the cupcakes, fit a piping bag with a large round piping tip (I use Ateco tip #808) and pipe frosting onto cupcakes. Finish cupcakes off with an Oreo.