Perfect Cupcake Frosting and Filling

(I haven't actually tried this yet, but it was on someone else's food blog and it sounded good, so I copied and pasted it.)

3 T Flour
1/2 C milk (whole milk is best, but I used non-fat when it's all I have and it's actually fine)
1/2 C real butter
1/2 C sugar (that's granulated sugar, not powdered sugar)
1 t vanilla extract
Whisk together the flour and the milk. Heat in a small sauce pan on medium heat. Whisk continuously until it starts to thicken. I think this is the critical point for any of you who have had problems with this recipe. I have a feeling people are under-cooking this part. Let it cook, while stirring, until it looks like pudding (you should be able to see the bottom of the pan when you stir it). Even though it's thick, you can still it through a mesh strainer (just whisk in the strainer to push the thick stuff through) and then let it cool completely to room temp. or chill it in the fridge. It needs to be cooled completely. If you don't let it cool completely, it will melt the butter and you'll have runny frosting.It an electric stand mixer, beat the butter and the sugar for a minute or two until well combined and fluffy. You'll want to use the whisk attachment on a stand mixer, not the flat paddle. Then while beating, add in the thickened milk mixture and the vanilla. Beat for 7 minutes. Yes, 7 whole minutes, maybe even 8. I know that seems like a long time, but that's when the magic happens! It will go from looking like a weird goopy mess to something thick, velvety smooth, and perfectly fluffy- like this:Use it to fill cupcakes or other pastries, or as a frosting on top. You can't go wrong putting this on just about anything :) And you'll want to use it right away. Don't put it in the fridge to use later because strange things happen. Just leave at room temp and eat it fresh.One batch makes enough to frost 12 cupcakes with BIG tops (like the one below, or larger). If you're just spreading a little on top, it will do 24.it in advance.