This is the best white cake I have ever tasted, so moist and has a wonderful flavor. It reminds me of a wedding cake. I use Madagascar Bourbon Vanilla which is in my opinion the best. I always use milk but if you prefer you can use water. You can add any flavor you want maybe a dash of almond if you like. It does take a little longer to bake I have gone as long as 40 minutes, it depends on your oven and pan size. It is very easy to work with, I cool mine on a wire cookie rack that I spray lightly with no stick cooking spray, it is very easy to frost and slices beautifully
Preheat oven to 325 degrees F (165 degrees C). Grease and flour an 11x13 inch cake pan, or two 9 inch round cake pans
Stir 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.