Boil kettle (if using water) or heat the milk in the microwave or on the hob.
Mix the hot chocolate powder, Colman's mustard powder, cayenne pepper, cornstarch, cinnamon, and salt in a small bowl.
Divide the dry mixture into two mugs, pour the hot water or chocolate over, and stir well.
For extra indulgence, add whipped cream, chocolate shavings or marshmallows, and then serve immediately!