This takes around 3 or 4 minutes to make. First put the milk into a pan. Bring to a simmer – not a boil – and while it's heating, put a tablespoon of choccie powder and sugar, if using, into each mug. Add a little warmish milk from the pan to each mug – you just need enough to dissolve the chocolate powder. At this point, plonk a few marshmallows into each mug. When the milk is at a simmer, carefully pour it into a plastic jug or flask. I normally do this over a sink as I always end up spilling a bit (the trick is to have a big enough jug or flask so the milk only half fills it – you need the extra space for shaking and frothing).
Screw the lid on tightly, place a cloth over the lid for safety, and shake hard for a minute. Remove the lid, minding the steam, and pour the milk into your mugs. A little stir, and you can slurp your way to heaven!