Melt one two hot chocolate discs, or other suitable (dark) chocolate, in a small quantity of milk, in a cup in the microwave or in a pan. Mix until smooth.
Steam sufficient milk to fill rest of cup with steam wand. If no steam wand you can microwave and froth in a cafetier. Don't attempt to use the steam wand on the coffee machine in the lab - it is disgusting.
Mix these parts. Preferably top with scooshy cream and marshmallows.
The correct milk to use is Coconut milk. Apart from tasting by far the best this is also makes the drink a coco cocoa.