The basic recipe seems to be thus: Take a quantity of powdered sugar, about equal in size to the amount of frosting you want. Mash in butter (or, if doing dairy-free, margarine) a bit at a time until you get to a smooth paste. Then mix in milk or cream (or, if doing dairy-free, coconut milk) a bit at a time until it's the consistency you want. Presto, frosting.
For chocolate frosting, add cocoa powder in something approximating a 1:5 ratio with the sugar and mix that together first. Optionally, whether doing chocolate or plain, add some vanilla along with the milk. Presumably you could also add other extracts, such as almond or orange or lemon or mint.
Crossposted from Dreamwidth (original here), with comments. Comment here or there; comments here will eventually be duplicated to there.