Bring the milk just to a boil. Put the chocolate in a blender and pour in the hot milk. Let sit for 10 to 15 seconds so the chocolate begins to melt and then cover securely, place a folded towel over the lid, and blend until completely mixed and frothy, about 30 seconds. (Alternatively, you can use a steamer to froth the milk. Add the chocolate, stirring until melted and well blended, and then steam the mixture again for a few seconds more.) Pour into two mugs and top each with a homemade marshmallow.