Grab a saucepan and add the following ingredients: flour and milk. Whisk the ingredients over a medium heat for a few minutes until the mixture begins to thicken. Once thickened, remove form heat and leave to cool before using. Add the vanilla, and set aside.
In a separate bowl, add the butter, salt and sugar. Beat the ingredients together for a few minutes until the mixture becomes creamy and light; scraping the bowl as you go.
Pour in the ingredients from the saucepan (remember, it should be cool) and beat for a further few minutes. At this point, it should look like whipped cream. Serve and enjoy!