Having a layer of foam on hot chocolate is as important today in Mexico as it was in ancient times. The chocolate is whipped to a froth with a carved wooden utensil called a molinillo and served in mugs. If it doesn't have foam, it's not alive. Mexican hot chocolate usually is imbibed with sweet breads such as those sold at Mexican bakeries. Mexicans like their sweet breads dry or slightly stale so they will soak up more chocolate.
Some people use water, some prefer milk.
For 1 cup, pour the hot liquid back and forth between two cups. For more, put the hot chocolate in a pitcher or deep-sided pot and use an electric beater or hand mixer. A cappuccino machine also can be used. Or try using a molinillo, available at many Mexican grocery stores. Holding the handle of the utensil between your palms, twirl it back and forth until the chocolate in the pitcher becomes foamy.