In a microwave, melt the whipped topping with the chocolate at 30 second intervals. Mix to fully combine.
If you want to keep this warm on Shabbos, transfer it to a plastic container and keep it on top of the urn until ready to serve. Note that you must place the container there BEFORE Shabbos, as liquids cannot be heated on Shabbos.
Recipe by Between Carpools at https://betweencarpools.com/absolute-best-dessert-cold-winter-night-hot-cold-perfect/