In a saucepan over medium heat, whisk together flour and milk until smooth and lump-free.

Continue cooking, stirring constantly, until the mixture thickens into a pudding-like consistency (about 5–7 minutes). Remove from heat and cool completely.

In a stand mixer, beat butter and sugar together for 5 minutes until light and fluffy.

Add the cooled flour-milk mixture one spoonful at a time, beating well after each addition.

Add vanilla extract and a pinch of salt, then beat the mixture for another 5–7 minutes until very light and whipped.

Use immediately or refrigerate in an airtight container. Bring to room temperature and rewhip before using again.