Seems like cooked frosting is as old as dirt... but I've only just recently come across it! It came to me on a piece of scratch paper, the "recipe" hastily and vaguely written in pencil. So with some research and guessing, I composited the Cooked Frosting recipe:
1 C Milk
3Tbsp AP Flour
Cook milk in small saucepan. Slowly add flour. Stir until thick, like pudding. (I only got it to "light sour cream consistency")
Once cool, beat with room temperature butter and Crisco. (You could use all butter. I think I might next time.)
Frosting should be fluffy - beat for a while until you get a good consistency.
Chill before icing.
I'm guessing you can use this as a nice base frosting and flavor as you like! I haven't tried anything though. But I do really like the fluffiness and thickness of it!