Preheat your oven to 350°F (176°C) and line a cupcake pan with cupcake liners.
In a large mixing bowl, cream the butter and sugar together until the mixture is light in color and fluffy, about 3-4 minutes. Be sure not to rush this step, as proper creaming will help give the cupcakes a lighter texture.
Add the sour cream and vanilla extract, mixing until well combined.
Add the egg whites in two batches, mixing until incorporated after each addition. Scrape down the sides of the bowl as needed to ensure all ingredients are well blended.
In a separate bowl, combine the dry ingredients. In a small measuring cup, mix the milk and water together.
Add half of the dry ingredients to the batter and mix until well combined. Then, add the milk mixture and mix again until fully combined. Add the remaining dry ingredients and mix until just combined, scraping down the sides of the bowl as necessary.
Fill each cupcake liner about halfway with the batter. Bake for 15-17 minutes, or until a toothpick inserted comes out with just a few crumbs.
Once done, remove the cupcakes from the oven and let them cool for 2-3 minutes in the pan. Then, transfer them to a cooling rack to cool completely.