Blend in brown sugar and confectioners sugar and simmer for 1 min.
Remove from heat. Whisk in ice cream, cinnamon, and nutmeg. Pour mixture into a plastic container, seal, and freeze (can be safely frozen for a few months).
Measure ~1 tablespoon of the Hot Buttered Rum batter in a mug then fill cup with boiling water.
Add rum to taste (or similarly leave out the rum). Stir and sprinkle with nutmeg. Serve hot