In a large mixing bowl, beat the shortening until smooth, about 1-2 minutes.
Add the softened butter, heavy cream/water and vanilla extract to the shortening. Beat for another 1-2 minutes
Sift half the powdered sugar into the shortening/butter mixture and mix on low until incorporated, then beat on medium-high for five minutes.
Sift in the remaining powdered sugar, mix on low speed until incorporated then beat on medium-high for an additional five minutes.
Store in an airtight container. Lasts 2-3 days at room temperature or up to a week refrigerated.
I always use Spectrum organic shortening to make this buttercream. It is not as greasy as traditional shortening and gives the buttercream a smooth and light texture. I have not tried the recipe with any other brand.
This simple vanilla buttercream is a great base that can be flavored in a variety of ways. Here are some possible flavor alternatives: