Make the batter: Get out a medium-sized mixing bowl and toss in all of the ingredients for the batter. Then use a spatula to smoosh and mix all of the ingredients together until they are thoroughly incorporated. Set this aside or place it in your fridge for later use.
Mix up the cocktails: Get out a mug and place the 2 tablespoons of batter, 2 tablespoons of dark rum, and heavy cream if using. Then pour half of the hot boiling water into the mug and stir it with a spoon until the batter is melted. Follow this up by pouring in the remaining boiling hot water.
Garnish the hot buttered rum with some nutmeg, or star anise and enjoy while it’s still hot.