Add the milk to a medium saucepan over low-medium heat. Allow the milk to warm almost to a boil. Add the cocoa, sugar and salt and whisk in a circular motion until fully dissolved, you’ll no longer feel the grainy bits of sugar at the bottom of the pan. Add the chopped chocolate and stir constantly until dissolved completely.
The mixture will appear quite light and loose at this point. Bring the mixture to a low boil while constantly whisking, about 3 minutes more, only lowering the heat a bit if the mixture rises too high. The hot cocoa should be darker now and thick enough to coat and wrap the back of a spoon, but not so thick that it isn’t able to be poured. Remove from the heat.
If using chili powder, add it now and stir to mix well. Serve hot!
Notes
The mixture will continue to thicken upon standing.If left to sit for too long, it’s not uncommon for a skin to form on the cocoa. Simply remove or stir vigorously to re-incorporate and serve.Consider adding vanilla bean or cinnamon and cloves.Top with whipped cream, if desired.