In your magically fantastic Kitchen Aid mixer, or an equivalent device, cream together your softened butter and shortening on a somewhat low speed until it is a smooth consistent mixture. While mixing at your lowest speed add the powdered sugar, cup by cup, going slow enough that it won't blow up into your face and hair and turn you into a ghost. Add your vanilla extract and milk and mix until combined. This will frost about 12 cupcakes or 1 small cake.