Brown the butter; heat butter in a small heavy duty saucepan over medium heat, stirring constantly; when butter turns to an amber/light brown color, remove from heat immediately (you should be able to smell the deeper more caramel flavor of the butter)
Pour butter into a medium heat-proof bowl (do NOT leave behind the sediment, it adds flavor); whisk in sugar, milk and vanilla
Immediately pour glaze over hot apple crisp and serve