For the white cupcakes

1. Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
2. Combine flour, baking powder and salt in a medium sized bowl. Set aside.
3. In a large mixing bowl, cream butter and sugar together until light in color and fluffy, about 2-3 minutes. Do not skimp on the amount of creaming time.
4. Add sour cream and vanilla extract and mix until well combined.
5. Add half of the dry ingredients to the batter and mix until well combined. Add the milk and water and mix until well combined. Add remaining dry ingredients and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Set aside.
6. Add the egg whites to a large mixer bowl and whip on high speed until stiff peaks form.
7. Gently fold about half of the whipped egg whites into the rest of the cake batter until mostly combined, then add the remaining egg whites and gently fold until well combined.
8. Fill the cupcake liners about halfway. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs.
9. Remove cupcakes from oven and allow to cool for 2-3 minutes, then remove to cooling rack to finish cooling.

For the vanilla buttercream

1. While the cupcakes cool, make the frosting. Add the butter to a large mixer bowl and mix until smooth.
2. Add about half of the powdered sugar and mix until well combined and smooth.
3. Add the vanilla extract, salt and 1 tablespoon of cream and mix until well combined and smooth.
4. Add the remaining powdered sugar and mix until well combined and smooth.
5. Add the remaining cream as needed and mix until smooth.
6. Pipe the frosting onto the cooled cupcakes. I used Ateco tip 844.