Pour the heavy cream into the fondue pot or into a small saucepan placed on the stovetop. Bring it to a simmer over medium heat.
When the heavy cream is bubbling lightly, lower the heat to medium-low and gradually stir in the chopped chocolate, mixing with a hand whisk or a heat-resistant rubber spatula until smooth.
When the chocolate is completely melted, stir in the vanilla extract.
Reduce the heat to the lowest possible. Serve immediately with your choice of dippers. If not using a fondue appliance, simply place the saucepan on a trivet and serve, or use a hot plate on the lowest setting to keep the fondue warm. Be careful - the fondue pot, hot plate, and/or saucepan will be hot.
Video
Notes
Results will vary depending on the chocolate brand you use. Start with the measurements listed here, then adjust them according to the results you get. If the mixture is too thick, add heavy cream, 1 tablespoon at a time. If it's too thin, add chocolate, 1 square (chopped) at a time.
To prevent the fondue from hardening, it's helpful to place it on a very low-heat fondue appliance or hot plate. You might also need to stir in an extra tablespoon of heavy cream if it starts to harden. Another trick is to add a tablespoon of oil (such as avocado oil) to the mixture as you melt it.
The nutrition info does not include dippers.
I would only keep the leftovers if there was no double-dipping involved. If you decide to keep the leftovers, store them in a sealed container in the fridge for up to 4 days. Reheat them covered in the microwave at 50% power. Reheat in 30-second sessions and stir the mixture after each session until heated through. Take care not to overheat, as chocolate burns easily. You might need to add a tablespoon or two of heavy cream to the mixture if it's too thick.