Add the following ingredients to a saucepan: milk, sugar, all-purpose flour, and salt. Heat on medium heat, stirring continuously. Keep cooking until the mixture becomes thick and then remove from heat.
Pour mixture into a bowl and chill in the fridge.
In a stand mixer, add your butter and beat. Keep beating until your butter is creamy, and then pour in the vanilla extract.
Gradually pour in your chilled mixture from earlier, mixing as you go. Serve and enjoy!