From Alton Brown:
* 2 cups powdered sugar
* 1 cup cocoa (Dutch-process preferred)
* 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. Store in an air-tight container.
Mix with warm milk, about 50% of milk/mixture.