Prepare the ingredients. Make sure the butter is at room temperature so it doesn't get lumpy.
Combine the ingredients. In a large mixing bowl, combine the butter, shortening, powdered sugar, milk (or cream), vanilla, and salt.
Initial mixing. Start blending at low speed until the ingredients are just combined.
Beat the mixture. Increase the speed to medium-high and beat until the mixture is very fluffy. This will take about 3-5 minutes.
Check the consistency. If the filling is too thick, just add a little more milk or cream, one spoonful at a time, until it's the thickness you want.
Use or store. Use immediately or cover and refrigerate.