In a large mug, combine all the ingredients and mix well using a small whisk, fork, or spoon.
Microwave the mug cake for about 90 seconds to 2 minutes until a toothpick inserted into the center comes out clean. Be careful not to overcook to prevent dryness.
Top the mug cake with toasted marshmallows and/or hot fudge sauce.
Allow it to cool for a minute or two, then dive in and enjoy the warm, chocolatey goodness!