Adapted from my mom, who uses what turns out to be the exact recipe on the side of a box of Hershey's cocoa powder. A surprising number of our old family recipes come from packaging.
Enough milk to fill 2 mugs (roughly 3 cups)
1/4 c. sugar (or more or less to taste)
1/4 c. cocoa powder (or more or less to taste)
1/4 c. water
1/2 t. vanilla
dash cinnamon (if desired)
In small saucepan, stir together water, cocoa, sugar, and salt. Bring to a boil for two minutes, stirring constantly. Stir in milk; heat until hot enough to qualify as 'hot cocoa' (do not boil), stirring every minute or two. Stir in vanilla and cinnamon; serve immediately. Top with marshmallows if you have any.