In a large saucepan combine the flour, sugar and salt. Stir to combine. Toast over medium heat for 30 seconds to 1 minute.
Slowly whisk in the milk, adding just a little at a time making sure it is completely incorporated before adding more.
Switch from the whisk to a silicone spatula and stir constantly over medium heat.
Continue cooking until the flour mixture has thickened and will drop off a spoon in clumps rather than pour off in a stream.
Remove from the heat and scrape into a bowl with the spatula. Cover the top with plastic wrap that is touching the surface, and allow to cool at room temperature until it is no longer warm to the touch.
In the bowl of a stand mixer fitted with the whisk attachment, beat the butter on medium-high speed until fluffy and almost white in color.
With the mixer running, add a spoonful of the flour mixture and allow it to beat in fully before adding the next.
Add the vanilla extract and continue to whisk on high until it holds stiff peaks.
Transfer to a piping bag or spread on cakes.
