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This is the one from Cooks Illustrated, adjusted a bit for us:

6 T cocoa powder
1 t cinnamon powder, optional
dash of salt
3 T honey (recipe calls for 4 T sugar, not honey)
1 cup water
3 cups milk
.5 cups half and half
1 tsp vanilla extract
(lots of whipped cream)

Combine water, cocoa, salt, cinnamon and honey in a saucepan and bring barely to a boil while whisking. Reduce heat, add in milk slowly. Bring milk to temperature, about 5-10 minutes or so. Just before serving, add in vanilla and half and half, and heat through for a minute. Pour into cups and top with lots of whipped cream.