Based upon various recipes online, but adapted to using the large container of my nutri-bullet. The secret is to finely blend everything together, it makes the mix dissolve easily.
1/3 cup coffee whitener (I used lite)
2/3 cup white sugar
2/3 cup instant slim milk powder
1/3 cup cocoa (the dark, cooking stuff)
Blend with the flat blades of your nutri bullet until it all mixes together - you should find that it is a consistent powder. If you don't have a nutri bullet, try 1/2 the recipe in a blender, pulsing to mix.
Use 1 heaping tablespoon per cup, add boiling water. For an extra special treat, add a splash of coconut milk for "cream"... it really adds a special flavor!
UPDATE, December 22
Just made a huge batch for my pantry...
done by parts:
4 parts sugar (measured before powdering)
8 parts skim milk powder
3 1/2 parts coffee whitener
3 parts cocoa
Much creamier, and not so sweet... really like this, although may up the cocoa to 4 parts too.