To rim the glasses: Put the syrup and chopped chocolate into 2 small dishes. Moisten the rims of 4 margarita or martini glasses with the chocolate syrup. Dip them in the Mexican chocolate until coated. Set aside.
Margarita: Combine all of the ingredients, except ice, in a pitcher. Working in batches, transfer the mixture to a shaker full of ice. Shake and pour into the prepared glasses. Serve cold.