We mixed the cocoa, sugar, salt and cinnamon together very carefully with a teaspoon. (If they aren't mixed well, Mother says the cinnamon floats on the top and the cocoa isn't so good.)
Then we each put our mixture into a saucepan and added the water. We cooked it slowly, stirring it all the time, until it got to be like a nice thick chocolate syrup. Then Mother had us add the milk slowly. We turned the fire low, and heated the cocoa till it was steamy and hot. (Mother told us not to let it boil.) When it was steaming hot, we added the vamilla and then beat the coca for a minute or two with a Dover egg beater. (Mother said that would keep it from getting scummy on the top. Robin likes the scum, but once he burnt his tongue on it, so I think Mother's way is best.)
Note: A Dover egg beater is a hand-operated mixer with two mixing blades. It was generally used to mix eggs, whip cream, and combine liquids. Oxo Good Grips makes a modern version that works relatively well, or you can use a whisk.
Mother had me put the four marshmallows in four cups and when we were ready, we poured the cocoa in on top of them. Of course people can use whipped cream instead of marshmallows if they have it, but most families don't, I've noticed. Anyhow marshmallow cocoa is very good.