Okay, so I didn't invent this frosting, I snagged it off the internet, maybe even off Martha Stewart's site. lol But I use it all the time and I try not to tell too many people what it's made of because of the looks I get.
7 Tablespoons all-purpose flour
1 Cup milk or Half and half
2 Teaspoons vanilla (add at the end of cooking above)
Combine those two ingredients and cook until thick. It's just like making creamed gravy except you don't put in oil or grease. Once this is thick, put into a bowl and cover with plastic wrap and cool till room temperature.
1 Cup real butter or margarine
1 Cup sugar
Combine these in a mixer and beat, beat, beat, until sugar dissolves into the butter. I generally let it mix for about 4 minutes.
Next, add in your flour/milk mixture and beat an additional 5 minutes minimum. You can tell by the mix when it's whipped enough as it will be a thick frosting at that time that holds peaks.
This is really a great frosting, cheaper to make than traditional buttercream frosting and not as sweet. It's become my favorite one to use and I use it every chance I get now.