Cream together the butter (shortening) and sugar alternative until fluffy. Meanwhile, sift together the flour, baking powder and salt in another bowl. In a third mixing bowl, combine together the egg whites milk and vanilla extract.
Now, in a large mixing bowl, if your butter and sugar alternative are already in one you can us that, add half of your flour mixture and half of the egg white mixture. Mix this gently and then add the remaining ingredients to the bowl. Scrap the sides of the bowl as needed. When the batter is blended you can transfer it to your prepped pans. Be sure to evenly disperse the batter throughout the pans. Using a rubber spatula is a good way to do this.
Please see recipe post for details.
As mentioned, you can make this a more moist and authentic white cake by using white shortening in this recipe. Use it with your butter and in place of half of the butter.
Use a fuller fat milk for a more moist cake or a lowest in sugar milk (like unsweetened almond milk) to keep the sugar down in this recipe.