Cupcake rescue needed!

Does anyone have an idea how to make icing or frosting that isn't very sweet?

I just made dark chocolate cupcakes that are sinfully delicious, but also rather sweet. I had an idea for orange icing, but anything with the usual amount of sugar in it is out.

So what can I spread over the chocolate that will fit and be tart rather than sweet? Can be refrigirated, and since I'm serving them tomorrow, I can buy ingredients as well.