In a small bowl, combine 4 tablespoons (60 ml) of almond milk, cocoa powder, and corn starch. Whisk until dissolved. Set the bowl aside.
To a medium saucepan, pour the remaining milk (240 ml), add brown sugar, a pinch of ground cinnamon, and a pinch of sea salt. Mix with the whisk. Heat the mixture until it starts boiling. Then reduce the heat add finely chopped dark chocolate bar, rum, and cornstarch milk mixture. Whisk until smooth and combined. Let cook for about 3 minutes until it thickens a little (stir occasionally) **.
Pour into a mug. Top with vegan marshmallows or coconut whipped cream. Sprinkle with cocoa powder, shredded chocolate, or ground cinnamon.
Serve immediately.