My good friend Sally C. writes in to ask:
You say you can freeze this? Wouldn’t it separate?
The answer is a.) Yes and b.) No. Classic buttercreams freeze very well, though it often happens that they look a little droopy or grainy when they’re thawed. This is easily remedied by, of course, beating them some more. Just make sure the buttercream is thawed completely before you do it, or you’ll have the same separating problems that can occur when you’re making buttercream and your butter is too cold.