However it usually does not make it to frost another cake. It is, I have learned through vast experience, just as delicious frozen as it is unfrozen. And totally scrumptious all by itself (sans the cake).
I made rugelach with my leftover icing. Guestimate the amount of flour
proportional to how much creamcheese/butter/sugar mix you have left in the icing.
Chill the dough, then roll out, spread with slightly warm apricot or raspberry jam, sprinkle with roasted chopped nuts and a bit of chopped chocolate. Roll up and cut, then sprinkle with coarse sugar and bake.