My guess is 2-3 teaspoons to 1 teaspoon cocoa powder, but it's obvious there is something else in there too.
A recipe for one cup of homemade instant cocoa is:
1/2 Cup of powdered milk
1 to 2 Tablespoons of sugar
1/2 Tablespoon baking cocoa
mix ingredients together and
add 1 Cup of boiling water
There are lots of cocoa recipes out there like the one suggested here, but I think Susan is wanting a chocolate powder mix, which goes in milk to make chocolate milk. I've never seen a recipe for this anywhere. It would be worth experimenting with, I think. I usually make chocolate syrup for them to put in milk.
I use this recipe for hot chocolate and I imagine it would be just as good with cold milk ... I also add just a splash of vanilla flavoring/extract ...
1 heaping teaspoon Hershey's Unsweetened Cocoa powder, 2 heaping teaspoons sugar and two cups (16 ounces) milk ...