Pour the instant pudding powder into a large bowl. Add the heavy cream whisk together until pudding powder is completely dissolved.
Add the eggnog, cinnamon, and nutmeg and stir with a large spatula until everything is combined. Taste and add rum, if using. Allow to chill in the refrigerator at least 1 hour before serving.
All nutritional values are approximate and provided to the reader as a courtesy. Changing ingredients and/or quantities will alter the estimated nutritional calculations.