Add cake mix, vanilla, egg whites, milk, and sour cream into a mixing bowl.
Mix in the stand mixer using the whisk attachment on low until the batter is fully combined, scraping the sides of teh bowl as needed.
Pour in two greased and floured 8 inch cake pans. Bake at 325 degrees Fahrenheit for 30 minutes, or until a toothpick comes out clean. Let cool for 5 minute in the pans, then turn onto wire racks to finish cooling.
Frost with your choice of buttercream and store in an airtight container in the fridge. In order to freeze, wrap your cake in plastic wrap and store in an airtight container.