Break the chocolate into large chunks and blend in a food processor until mealy.
Add the cocoa, sugar, powdered milk (whichever kind you’re using), salt, and cornstarch to the food processor. (If you don’t have a food processor, finely grate the chocolate instead — then stir in the rest of the ingredients by hand)
Blend until combined.
Scoop the mixture into a jar (make sure you scrape around the edges of the food processor bowl to dislodge any chocolate that may be stuck there) and screw on the lid.
For one serving of hot chocolate:
2 heaping tablespoonshot cocoa mix
In a small pot combine the milk and hot cocoa mix.
Whisk together gently over a medium heat until well mixed and steaming. Pour into a mug and serve.