Combine the mix. In a large bowl, whisk together the powdered sugar, dark cocoa powder, unsweetened cocoa powder, powdered milk, vanilla powder, salt and cornstarch until evenly combined.
Store. Transfer the Hot Cocoa Mix to an airtight container and store in a cool, dry place for up to 6 months. Store the chocolate chunks and marshmallows separately, if using.
Make one mug. Add 2 1/2 heaping tablespoons Hot Cocoa Mix to a mug and whisk in 1 cup hot water or steamed milk until smooth.
Serve. Top with chocolate chunks, marshmallows or your favorite toppings and serve immediately.
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Notes
Store Hot Cocoa Mix in an airtight container in a cool, dry place for up to 6 months.
Stir or shake the mix before scooping because the powdered ingredients can settle during storage.
If you cannot find powdered vanilla, substitute powdered vanilla coffee creamer or add vanilla extract to the prepared cocoa instead.
For dairy-free Hot Cocoa Mix, substitute a non-dairy powdered milk or powdered creamer.
Whole milk powder makes a richer cocoa, but nonfat powdered milk works well.
Natural, dark or Dutch-process cocoa powder can be used depending on the chocolate flavor you prefer.
Hot water works well because the mix contains powdered milk. Use steamed milk for an extra-rich cup.
Keep marshmallows and chocolate chunks separate from the base mix for easier measuring and fresher toppings.