In a large metal bowl, over a double boiler, whisk together the egg whites and granulated sugar. Stir constantly until the mixture reaches approximately 115*F andthe sugar grains have completely dissolved. Remove from the double boiler and transfer to the bowl of a stand mixer, if it isn't already. With the whisk attachment on the stand mixer, whip on high speed until it is thick, glossy, and white. It should reach firm peaks. While it is mixing, sift the powdered sugar. Once the egg whites are at stiff peaks, add the powdered sugar, all at once and mix on the lowest speed until it is incorporated. After a minute or so turn the mixer to med-high and whip until it is completely incorporated. Make sure the butter and shortening are at room temperature and they are the same consistency. Add it in 2-3 additions, but do them quickly. Switch to the paddle attachment and continue mixing on low to medium speed until smooth. Woodland Bakery has a wonderful video tutorial on this recipe @ www.woodlandbakeryblog.com