Prepare your drinking mug by warming it up. Make sure you're using a heat-resistant mug. Pour piping hot water into it and cover with a small plate. Once warmed, empty the hot water and add brown sugar and hot milk (or fresh, hot water if you prefer). Stir until the sugar dissolves and then add rum. Gently float the butter atop the drink and sprinkle some nutmeg over that.
Courtesy Stephen Phillips