Preheat the oven to 325 degrees. Line two cupcake tins with paper liners.
To make the cupcakes, in a large bowl, whisk together the egg whites, sour cream, oil, buttermilk, and vanilla. Add the cake mix, and mix until just combined (be careful not to over-mix). Scoop the batter into the prepared cupcake tins, filling 2/3 full.
Bake 20-22 minutes, or until the tops spring back when touched lightly, and a toothpick inserted into the center comes out clean. Let cool completely.
To make the frosting, combine all ingredients in a medium bowl. Beat with an electric mixer until well blended and smooth. Frost the cupcakes with the cream cheese frosting.