In the bowl of a stand mixer with the paddle attachment, whip the butter and shortening together until they are combined and fluffy.
Add in the powdered sugar, salt, and 3 tbsp of buttermilk.
Whip together until light and fluffy, adding in the additional buttermilk if necessary to get a spreadable consistency. Whip for at least 5 minutes to make sure everything is incorporated and fluffed up!