Meanwhile, in a small bowl whisk together cake mix and cinnamon then whisk in milk and stir until well blended.
Pour mixture over melted butter in mug (don't stir). Drain off 2 Tbsp liquid from peaches (about 2/3 of the liquid), then pour peaches over top of cake mix (don't stir).
Microwave on 50% power for about 3 - 4 minutes, until desired doneness (time may vary depending on each microwave oven and wattage).
Allow to cool slightly (it will be ridiculously hot) then optionally top with vanilla ice cream and a light dusting of cinnamon.