Preheat the oven to 300ºF. Line a baking sheet with parchment paper and spread the milk powder in an even and thin layer.
Bake for 10 minutes, or until golden brown. The milk powder will be stuck together so place in a food processor and pulse until broken up.
In a bowl, add the milk powder, sugar, cocoa powder, cornstarch and salt and mix until combined. Cover or place in zip loc bag.
To make hot chocolate, heat 1 cup of milk in a small saucepan over medium heat until it starts to steam and bubbles appear around the edge of the saucepan. Add 3-4 tablespoon of the mix and simmer, whisking constantly, for 2-3 minutes longer. Pour into a mug and serve.
Notes
Bake the dry milk powder to add a slight caramel flavor to the hot chocolate.
If you want a super fine cocoa mix, throw all of the ingredients in the food processor and pulse until everything is combined.