In a mixer, with a paddle attachment cream together the butter and half of the powdered sugar.
Scrape the bowl down and add in the shortening and the remainder of the powder sugar. Mix with he paddle attachment until fully combined and smooth.
Change the paddle attachment to the whip attachment, and scrap down the bowl so all the sugary fat is off the sides of the bowl.
Turn on mixer to medium speed. Slowly add in half of the the egg whites. Mix until combined and then turn mixer to high. Whip for 10 to 15 seconds.
Turn off the mixer, and scrap the sides of the bowl again. This is ensuring all of the ingredients are properly incorporated.
Turn on mixer to medium speed. Slowly add in the remainder of the the egg whites and the vanilla. Mix until combined and then turn mixer to high. Whip for 10 to 15 seconds.
Remove from mixer stand and use accordingly.
