Combine the butter and shortening in the Kitchen Aid (or stand mixer) bowl with the paddle attachment
Whip on medium to high speed for about 3 minutes.
Scrape the sides and bottom of the bowl to make sure it is evenly mixed and add the salt. Mix well.
Stop mixer and add the sifted confectioners sugar all at once.
Mix on low speed until incorporated , scrape the bottom and sides of the bowl and then mix on high speed for 3 minutes.
Scrape the bowl again and then while mixing on low speed slowly drizzle in the heavy cream.
Increase speed to high and whip for another 5 minutes
Add the flavor extracts and then you are done!
The icing will gain volume almost to the top of a 6qt Kitchen Aid bowl and get very white. (*If you are using a hand beater, the volume will not get as high since the hand beaters cannot incorporate air as efficiently as the Kitchen Aid or stand mixers)
Recipe by Gretchen's Bakery at https://www.gretchensbakery.com/russian-ball-tips-and-ruffle-tips-review/