Combine whole milk and dark chocolate bar (diced up) to a small pot. Let simmer over low heat and stir occasionally until the chocolate is melted. Set aside the hot chocolate.
Combine heavy whipping cream and ground cinnamon to a mason jar, then shake vigorously until cream has thickened to desired whipped texture
Add mezcal, creme de cacao and chocolate bitters to a mug.
Stir.
Add hot chocolate to the mug until the mug is about 3/4 of the way full.
Add whipped cream to fill the rest of the glass.
Garnish with grated chocolate.