In a small saucepan over low heat, heat the milk until scalding but not boiling.
Add in the chopped chocolate, agave, cocoa powder, and vanilla extract. Stir until chocolate melts fully. Remove from heat and stir in peanut butter.
Microwave 1 tablespoon of peanut butter for 15-20 seconds in a small bowl, then drizzle up and down the sides of a mug. Pour in hot cocoa, then top with marshmallows. Torch marshmallows using a small torch or by heating under broiler for 30 seconds!