Line a muffin pan (12 muffin spaces) with baking cups, then set to the side.
In a glass bowl, combine the semisweet chocolate chips, heavy whipping cream, instant espresso powder, chipotle powder, cinnamon, and salt.
Microwave the mix for 3 minutes (or until smooth), stirring half way through.
Pour the chocolate mix evenly into the baking cups and place the muffin pan into the freezer. Allow the chocolate to freeze (2 to 3 hours). At this point, you can remove the baking cups from the pan and place them in a freezer bag (you will have extra for future servings if less than 12 servings needed).
Take four the chocolate baking cups and remove the paper. Place the chocolate disks into microwave-safe mugs (one per mug).
Pour the milk over the chocolate, then microwave the mug for 2 minutes, stirring once at the 1 minute mark.
Top with whipped cream (optional), sprinkle with additional chipotle powder (optional), and serve.
Note - the recipe is for 12 servings, but this is mainly to support a 12-cup muffin pan for the chocolate disk portion of the recipe. If you don't need 12 servings, simply save the disks for future use. Use 3/4 cup of milk per serving needed. Feel free to increase the amount of chipotle powder used if you want a smokier and spicier Aztec hot chocolate.