You don't see vanilla and milk because I forgot them in the picture.
Cream the butter and shortening until very creamy. About 3-4 minutes.
Add the powdered sugar and blend until starts to look crumby.
Add the milk and vanilla. Whip till creamy and fluffy on medium high for 5 minutes. You can use any flavor you want.
It should look very airy and light. You can store this covered in the fridge for about two weeks. Makes about 3 cups. Great for filling a cake or just frosting the top. You can tint it any color.