Combine the milk, cocoa powder, sugar, molasses, ginger, cinnamon nutmeg and allspice in a medium saucepan. Cook over medium heat, stirring occasionally, until the sugar, cocoa powder and spices are dissolved and the milk is steaming, about 5 minutes. Do not let the mixture come to a boil. Whisk in the chopped chocolate a little at a time until smooth. Divide among 6 to 8 mugs and garnish with whipped cream and mini marshmallows.