Toss all ingredients into a smaller, round (mini) crockpot. Stir in the cocoa powder, put lid on and let it cook for 2 - 2.5 hours on high. Stir once or twice to incorporate melted chocolate.
Around the 2 - 2.5 hour mark, remove lid and stir to ensure all combined. Pour into a mug, top with marshmallows or whipping cream.