Cocoa mix recipe--your favorite cocoa recipe plus 1/3 C. dried milk for each cup. Mix very thoroughly. I prefer to package individual servings in sandwich bags. Use hot, not boiling water to mix (boiling water may cook/curdle the milk, making nasty lumps), and mix with maybe 1/3 C. water to make a paste before adding the rest of the water.
I like to use high quality cocoa or cocoa mix (the kind you add to milk) like Ghiardelli, Dagoba or, especially, Green & Black. Read the ingredients on the supermarket stuff like Swiss Miss and you'll see why I avoid it. For my own use, I prefer to make my own mix from scratch (cocoa, sugar, dried milk) so I can cut back on the sugar. That takes more pre-mixing than using a cocoa mix, to get out the lumps of powdered cocoa. It's easier with the cocoa mix.
Important--Experiment at home and make no more than a cup of cocoa at a time until the mix is as you like it. You may want to use more or less dried milk, more or less cocoa, more or less sugar. Some like a few grains of salt added. I like to add a little cinnamon. My son #3 likes a dash of cayenne pepper. YMMV!
For mocha, add a little instant coffee. If you don't want mocha, use Sarah's recipes without the coffee