• In a small bowl, combine the dry milk, cocoa powder, powdered sugar, cornstarch, and cinnamon, if using.
• Stir with a whisk to combine completely and break up any dry lumps.
• Stir in the dark chocolate.
• Transfer to an airtight container and store at room temperature for up to 3 months.

To serve hot chocolate mix:

• Place 1/4 cup of the mix into a large mug and pour 1 cup very hot water into the mix.
• Stir well with a spoon and top with marshmallows or whipped cream, if desired.