Bake your favorite chocolate cake (or whatever you want to frost!) and let it cool.
In a small saucepan, whisk flour into milk and heat, stirring constantly, until it thickens. Remove from heat and let it cool to room temperature. (If I’m in a hurry, I place the saucepan over ice in the sink for about 10 minutes or so until the mixture cools.) Stir in vanilla.
While the mixture is cooling, cream the butter and sugar together until light and fluffy. Then add the cooled milk/flour/vanilla mixture and beat until it all combines and resembles whipped cream.
Grab a spoon and taste this wonderful goodness. If there is any left after your taste test, spread it on a cooled chocolate cake. Cut yourself a piece and put it on a pretty plate. Grab a fork and prepare to experience the most divine pairing you can imagine. This frosting on chocolate cake is to die for. Sure, the recipe sounds strange — it has flour in it — but it’s sublime. Try it, you’ll see. You’ll love it so much you won’t go back.