1. Melt butter in a large pot over medium heat. Blend in brown sugar and confectioner's sugar. Remove from heat, and whisk in the ice cream, cinnamon and nutmeg. Pour mixture into a plastic container, seal and freeze.
2. In a coffee mug, measure 1 tablespoon Hot Buttered Rum Butter and 1 fluid ounce rum, then fill cup with boiling water. Stir and sprinkle top of drink with nutmeg.