Grab a bowl and add the following ingredients: flour, salt, sugar and cornstarch. Mix well, and then gradually add the milk.
Once smooth, strain your frosting and add the mixture to a pan, over a medium heat. Heat for around 5-8 minutes, stirring throughout; at this point it should be boiling. Remove from heat and leave to completely cool.
Once cool, add vanilla extract and beat for around 30-60 seconds. Gradually add the butter; beating as you add it. After a few minutes, the frosting should be fluffy and airy.
Chill your frosting for around an hour in the fridge before using, to help firm it up. Serve and enjoy!