Over a double boiler, in a large metal bowl whisk together the granulated sugar and the egg whites stirring constantly until the mixture reaches approximately 115 degrees F and the sugar grains have dissolved and are not longer gritty to the touch.
Pour this mixture into the metal bowl of your mixer with the WHISK attachment and whip on high speed until it is thick, glossy and white in color. The peaks should be as firm as firm can be.
In the meantime you can sift your confectioners sugar, and once you have achieved STIFF peaks meringue, add the confectioners sugar all at once with the mixer on very low speed to avoid having a snow shower of sugar all over your kitchen!Once all of the sugar has been incorporated you can turn the mixer speed to med-high until it is all nicely mixed in.
Be sure your butter and shortening are at room temperature and they are the same consistency. Add it in about 2-3 additions, but FAST Additions. 1-2-3 fast fast!
Turn the speed up to high and let it mix smooth. Add the vanilla extract at the end, and you are DONE! Best Buttercream EVER!
add the melted cooled chocolate and mix in with a spatula until well blended