Preheat oven to 300 degrees F.
Spread milk powder in a thin, even layer on a parchment-lined sheet pan and bake for 20 minutes. Remove from oven and cool. Odds are good the milk powder will be stuck together in lumps and if that's the case, simply pulse it in a food processor a few times. (Note: Toasting the milk powder is technically optional but personally, I think the flavor is worth it.)
Combine all ingredients (except hot water) in an airtight plastic container or glass jar and shake to thoroughly combine. Tightly sealed, the powder will keep nearly indefinitely in the pantry.
For one serving, heat 6 ounces of water to the boil then remove from heat. Spoon 1/4 cup of the cocoa mix into a heavy mug and whisk in 2 ounces of the water to create a paste then slowly whisk in the remaining hot water. Allow to sit about 30 seconds because otherwise you'll scald your face while attempting to chug the deliciousness.
If, like me, you only have kosher salt, simply blend a few tablespoons to a fine dust.