Mix together with a small whisk or fork. If you don't have a huge oversized mug, split the batter into two cups and cook them separately.
Microwave on high for 1 minute for small mug and 1 minute 30 second for large. Check doneness. Microwave for 30 second intervals until done. I made mine one giant serving and it was done at 2 minutes.
While cake is cooling make frosting. Beat heavy cream, coffee and sugar on high speed until peeks form. Top on cake when cooled. You can also add a little bit more cinnamon over frosting.