Ingredients:

2 cups powdered sugar
1 cup cocoa (Dutch-process preferred)
2 1/2 cups powdered milk
1 teaspoon salt
2 teaspoons cornstarch
1 pinch cayenne pepper, or more to taste
Hot water

Combine all ingredients in a mixing bowl and incorporate evenly. In a small pot, heat 4 to 6 cups of water. Fill your mug half full with the mixture and pour in hot water. Stir to combine. Seal the rest in an airtight container, keeps indefinitely in the pantry. This also works great with warm milk.

Source: foodnetwork.com
Recipe courtesy Alton Brown