Take a standard flour-based frosting (which is delightful) and sub any citrus juice you like for the milk. You'll basically end up with a citrus pudding that you then cool and whip butter Into. Day-um, it is good. This recipe makes about 2½ cups of frosting. Double it so you can eat a bunch right out of the bowl. Also, before you whip the butter into it, you could use it as a filling for donuts or cupcakes. You should probably do this and then tell me all about it. If you're using fresh-squeezed citrus, by all mean whip in some of the zest to underscore the goodness.