Put the vegan butter and vegetable shortening into a large mixing bowl or the bowl of a stand mixer. Begin to whip together either with a hand mixer or the whisk attachment of the stand mixer.
Once they are smooth, start adding the powdered sugar, a little at a time. Continuing to beat together.
When it starts to get thick, add the non-dairy milk, continuing to whip together.
Then add the remaining powdered sugar.
Once the frosting is very smooth and fluffy, add the vanilla and salt. Whip again until fully combined, very thick and fluffy. If you are making vanilla frosting, you're done!
If you are making chocolate or strawberry, now is the time to add either the cocoa powder or freeze dried strawberry powder! Whip again until fully combined.
If you are making peanut butter frosting, you want to add the peanut butter in step number one, with the vegan butter and vegetable shortening. Whip that to combine. Then proceed with steps 2-5!