Melt butter in a large pot over medium heat. Blend in brown sugar and powdered sugar. Remove from heat and whisk in ice cream, cinnamon and nutmeg. Pour mixture into a plastic container, seal and freeze.
In a coffee mug, measure 1 T. Hot Buttered Rum Batter and 1 oz. of rum; then fill mug with boiling water. Stir, and sprinkle top of drink with nutmeg.