Preheat the oven to 350 degrees. 
Line a cupcake pan with liners (or spray with non-stick spray) and set aside.  
Cream together the softened butter and sugar until well combined.
Add in the separated egg whites,  the milk, vanilla and oil.  Thoroughly mix the ingredients - stopping the mixer and scraping the sides occasionally to ensure everything gets mixed in.
In a separate bowl, combine the dry ingredients - flour, salt and baking powder - and whisk.
Combine the dry ingredients with the mixing bowl containing the butter and liquid ingredients.  Stir on low speed for about a minute.  Stop the mixer and scrape down the sides.  Continue to mix on medium speed for about another minute until the mixture is well combined.
Fill each cupcake cavity until about ⅔ full.  Bake for approximately 14-16 minutes or until slightly golden brown and the middle of each cupcake is set.  
Allow to cool to the touch before decorating.