Preheat oven to 300ºF. Line a half-sheet pan with parchment paper.
Spread the milk powder in a thin, even layer about 1/4-inch thick on the prepared half-sheet pan. Place the pan in the oven and toast the milk powder for 20 minutes. Remove from oven and let cool. If the toasted milk powder is stuck together in clumps, you may need to pulse it in a food processor to return it to its original, powdery state.
Combine all of the ingredients in a large airtight container. Secure the lid and shake vigorously to combine.
To make one serving, bring 6 ounces of water or milk to a boil and remove from heat. Put 1/4 cup of the cocoa mix in a mug, add 2 ounces of the hot liquid to the mug, and whisk into a smooth paste. Add the remaining liquid and stir again. Serve hot.