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.