Comedians often mine life's little inconsistencies for humor, such as hot dogs sold in dozens and buns sold in bags of eight. Bakers know that feeling as well, because -- for example -- your favorite icing recipe seldom makes the exact amount you need to go with your favorite cake. Making lots of icing and refrigerating the leftovers is a pragmatic solution to the problem, but it leaves your buttercream unusably stiff. Before you can ice your next cake, you'll need to soften the icing.
Take the container of buttercream from your refrigerator. Scoop as much as you'll need into a mixing bowl, and return the rest -- if any -- to the fridge until your next decorating project. Break up the icing into smaller pieces, to speed the softening process.
Rest the icing at room temperature for 30 to 45 minutes, or until it's workable. Cream it with a wooden spoon, or whip it in a stand mixer, until the friction softens it to a spreadable consistency.
Add clear corn syrup, a few drops at a time, if the buttercream remains persistently stiff. The corn syrup will loosen and lubricate the buttercream, without altering its texture appreciably. Beat the buttercream until the corn syrup is fully distributed through it, and there are no remaining stiff areas in your icing.
Spread or pipe the icing on your cake, as you would if it was freshly made.