2Stir together the white cake mix, flour, sugar, and salt in a large bowl until well mixed. Pour in the milk (or water),sour cream, vegetable oil, and vanilla extract, and egg whites, and beat with an electric mix on low until all the ingredients are mixed and moistened about 4 minutes
3. Pour the batter into the prepared cake pan, and bake in the preheated oven until the top is a light golden brown and a toothpick inserted into the center of the cake comes out clean, about 35 minutes. Allow to cool before frosting.