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.

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.

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. Continue to beat until the mixture becomes light and fluffy and resembles whipped cream.