Start by making the whipped roux frosting.
To a small saucepan, add the milk and flour. Turn the heat up to medium, and whisk the flour into the milk until it is smoothly incorporated and there are no lumps (or almost no lumps!)
Keeping the pan over medium heat, slowly whisk in the sugar, followed by 1 tsp of the vanilla extract.
Keep whisking over medium heat until the mixture comes to a boil.
Once boiling, boil for one minute, whisking the whole time. The mixture will get thick like pudding.
After one minute, take the pan off heat, and transfer the roux to a (preferably glass) bowl.
***If there are lumps in your roux, strain it before you transfer the roux to a bowl.