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 2 1/2 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.
¾cupcitrus juice(I used Nelly&Joe's key lime this time)
¾cupsugar(to taste, I added an extra 1 1/2 tablespoons this time)
1whole egg(optional, but I added it for richness and to tame some of the Tang)
3Tablespoonsall purpose flour
¾teaspoonsalt(it needs it--but use it to taste. Just don't leave salt out. Ever).
¼teaspoonvanilla(optional. I used it for a hint of Mellow)
1 ½sticks unsalted butter, soft but not melty
Instructions
Put the juice, sugar, egg, flour and salt into a heavy bottomed sauce pan.
Heat over medium-high heat, whisking quickly and constantly, until the mixture comes to a boil.
Let boil for about 1 minute, still madly whisking, and then pour through a fine-mesh strainer into a bowl.
Press all of it through the strainer with a flexible rubber or silicone spatula, making sure to scrape any that is sneakily sticking to the underside of the strainer.
Press plastic wrap onto the surface of your pudding (that's what it is, after all), and cool to room-ish temperature in the fridge.
Scrape your pudding into the bowl of your stand mixer fitted with the whip attachment.
With the mixer on medium, add the butter, a tablespoon at a time, incorporating each addition before adding the next. Scrape the bowl and beaters often.
When all the butter is in, whip in the vanilla and beat on high speed for a minute or two to lighten the texture just a bit. Taste and add additional salt, if necessary.
Notes
I found that the flavor mellowed and matured in the fridge overnight. You can frost your cake immediately, but I'd recommend waiting until the following day to serve it. Also, you can just refrigerate the frosting immediately and then ice and serve your cake the next day. Make sure to re-whip the frosting once it comes to room temperature.