Simmer milk and vanilla over medium heat. Slowly whisk in flour until fully incorporated and there are no lumps. Cook until mixture thickens to the texture of yogurt. Remove from heat, but continue to stir for a minute or so since the it will continue to cook as it cools.

Cook milk mixture completely. (If you add it to the butter mixture while still warm it could melt the butter.

Cream butter and sugar together on high until fully combined and fluffy. Approximately 5 minutes.

Slow mixer down and slowly incorporate milk mixture into the butter-sugar mixture. Beat on high until light and fluffy. Approximately 5 minutes.