Whisk all the ingredients together in a large cocoa or coffee mug until smooth.
Microwave the cake on high until puffed, about 2 minutes, watching carefully as microwaves can vary in strength. If desired, drizzle chocolate syrup or hot fudge sauce on top.
Scoop cake onto a plate or in a bowl and serve immediately with a scoop of ice cream, if desired, but which I highly recommend.
For a PB Variation: try subbing peanut butter for the Nutella. If you aren't a fan of either, simply leave it out. Serving: err on the side of under-microwaving slightly and eat it immediately. The texture gets a bit spongy and weird upon sitting.