In a small saucepan, whisk the flour and sugar together until the flour no longer has clumps. Add the milk and continue to whisk while placing the pan over medium heat. The mixture will burn easily so stay on top of it. Continue to heat until it starts to bubble and thicken. Whisk vigorously for another minute while the mixture bubbles, remove from the heat and continue to whisk for another 2 minutes.

Pour into a bowl and cover with plastic wrap so that the plastic wrap touches the surface of the pudding mixture. This will prevent a skin from forming.

Wait until pudding mixture comes to room temperature. This will take a few hours so I always start this process before I start my cakes. You can chill it in the fridge to speed up the process after the pudding mixture has been sitting out for an hour.

Slice the vanilla bean lengthwise and scrape out the seeds. Save the pod for another use.

In a standing mixer, add the room temperature butter and the seeds you scraped from the vanilla bean. Whip on high for 10 minutes before adding the pudding mixture one big spoonful at a time until it has all been incorporated.