In a large bowl cream together the confectioners’ sugar, cocoa powder, salt and butter (blend at low speed so the sugar doesn’t poof up all over you and everything within two feet of the bowl). Add the vanilla, and 4 tablespoons milk or cream. Keep blending until everything is smooth, and add more milk as needed to reach the desired consistency.
The frosting can be make a day ahead and left at room temperature, covered with plastic wrap or in a plastic container, of you can make it several days ahead and refrigerate it, though you should definitely let it come to room temperature before attempting to spread it on a cake. You will also want to give it a good beating with a spoon or an electric mixer after it does warm to room temp, which will make it smoother and easier to spread.