Hello Jane,
Following is a Hot Cocoa from Alton Brown's Good Eats program that I use. I tried a few others and I always go back to this one.

HOT COCOA MIX
by Alton Brown

2 1/2 cups powdered milk
1 cup Dutch Processed cocoa powder*
2 cups powdered sugar
1 tsp salt
2 tsp corn starch
Optional: a bit of cyanne pepper if wanted

Dump everything in a container with a lid or a one gallon plastic zip thpe plastic bag. Lid the container or zip up the plastic bag and shake until no evidence of streaks of cocoa powder or powdered milk.

TO SERVE:
Put pot of water on stove to heat up just under a boil (if the water is too hot the mix can curdle. I put a cup of water in the microwave for one minute then pour about 1/3 of it into the cup with the cocoa mix and wisk until smooth then add the rest of the hot water and stir. It it is not as warm as you want it to be just pop it back into the microwave for another 15 seconds or so.)

Choose mug desired
Fill the mug 1/3 full with cocoa mix
Pour about 1/3 cup of the heated water into mug
Wisk until smooth consistancy
Add more heated water to desired fill
Stir and enjoy!

*NOTE: I don't like my cocoa so sweet so I add 1/3 more cocoa powder to the mix.

VARIATION:
He gives the option of adding cayenne pepper to the mix and it tastes pretty good but I don't like it all of the time so I leave it out and just put a pinch of the cayenne pepper in my mug when I crave it that way.

I usually give this as gifts for Christmas. Also drinking a cup before bed helps me fall asleep easier.