Whisk together the flour, sugar, salt, and milk in a small saucepan over medium-low heat. Once it comes to a soft boil, allow the mixture to boil for 30-60 seconds or until it thickens enough to reach the consistency of a loose pudding.
Remove the mixture from the heat and decant into a container. Cover with a piece of parchment to prevent a skin from forming and allow to cool to room temperature.
Once the pudding has cooled, continue making the frosting.
In a stand mixer fitted with the paddle attachment, whip the butter on high speed for 5 minutes until pale and fluffy.
Turn the mixer to a medium speed and dollop 1 tablespoon of the pudding into the mixer. Allow the pudding to fully incorporate into the butter and then continue adding the pudding to the butter slowly until finished.
Once all the pudding has been added, add the vanilla and beat the mixture on high speed until light and fluffy. Turn the mixer down to medium-high and slowly drizzle in the whiskey. Beat on high again until fluffy.