I like to make ganache--heat cream to simmer and whisk in chopped chocolate (use really good chocolate) till it melts. You can dip the cupcakes into the warm ganache and let it harden on the cakes (It has a pretty, shiny surface that way) or let the ganache cool in the refrigerator till it's spreadable and spread like icing. (I imagine you could pipe that, too, but I've never tried it.)
There is a wonderful chocolate icing on Joe Pastry's website: joepastry.com. It is really easy and looks great piped on chocolate cupcakes with a large star tip.
Sorry! I just saw the "less sweet" part of your question. You can whip the ganache suggested by drbabs. Place the bowl with the warm ganache into another, larger bowl with ice cubes in the bottom. Use a hand mixer to whip. Be careful not to over do it or it will curdle.
ganache made with a darker chocolate would definitely work. Or you might try this recipe by smitten kitchen, which incorporates sour cream, so that it's less sweet.
Alternatively, just use less powdered sugar in whatever recipe you are using. Frosting, unlike the baked good itself, lends itself to tweaking based on your taste preference.