Heat the milk in a small pot over medium-low heat.
Once the milk is almost at a simmer, whisk in the cacao powder.
Stir in the peanut butter and chocolate, stirring until the chocolate has melted.
Add the vanilla and cinnamon, if using.
Whisk in maple syrup to taste, if using—my oat milk was naturally sweet enough, so I did not need maple syrup.
Pour into a mug, then top with vegan marshmallows if you wish.