Waldorf Frosting for Red Cake

This is the original frosting for Red Cake. It is also called boiled-milk frosting or Ermine frosting.

1 c. whole milk (I used a combo of 2% and heavy cream in abt 9:1 ratio)
5 tbsp flour (40g)

pinch of salt
1 tsp vanilla
1 c. butter
1 c. powdered sugar, packed and sifted

1. Over medium heat, whisk flour and milk in a small saucepan and heat to a simmer, stirring frequently until it becomes very thick and almost puddinglike.
2. Remove from heat, whisk in vanilla and salt. Pour into a bowl to allow it to cool completely. Put plastic wrap on the surface to keep a skin from forming!
3. Use a mixer to cream together butter and sugar until light and fluffy, scraping the sides of the bowl occasionally, about 5 minutes. With the mixer on medium, add the cooled flour mixture a little bit at a time. 
4. Continue to beat until the mixture becomes light and fluffy and resembles whipped cream.