A couple of weeks ago I attended a cooking class focused on cupcakes. I'm planning to try a couple of basic recipes (vanilla cupcakes and buttercream) this week-end.
The problem is, they were given as separate recipes. So, while I'll get (about) 18 cupcakes, the recipe for buttercream calls for 9 oz. of butter.
It sounds a bit much for 18 cupcakes, especially because my other cupcakes-and-buttercream recipe only calls for 3.5 oz.
I tried that months ago and I remember having more than enough icing for 12 cupcakes - as in "having to eat a couple of spoonfuls straight from the bowl"

Any suggestions? Also, does anyone know how long buttercream keeps?
(My apologies if I've used the wrong terminology and/or measurements)