It always pays to make extra. If you’re making layers for a birthday cake, it’s just as easy to make four as it is two (provided you have the pans). The same thing goes for buttercream. If I’m going to the trouble, why not make a double batch? For it too freezes very nicely and keeps well in the deep freeze. Even frozen butter, however, will pick up odors and off-flavors if left for too long, so I’d only keep a frosting made from it for a month at the outside (and then never in the same freezer as, say, frozen fish). However one of the secrets to regular great baking is having these kinds of things lying around. In the same way that great cooks always have a little homemade stock, a few frozen cubes of demi glace, or some fresh herbs stowed here and there around the kitchen, great bakers rely on their secret stashes of cake, doughs, icings and fillings. It takes very little extra work to make an extra batch of say, a tart dough, but the rewards can be great when you’re called upon to produce a last-minute miracle.