In a small saucepan, combine milk, heavy cream, cinnamon syrup, and cayenne pepper. (Remember, you can always start with just a pinch of cayenne, and add more later if you want an extra kick.)
Warm over medium heat, stirring occasionally until the mixture is warm but not boiling. Turn off heat.
Cut dark chocolate into small pieces and add to the pan. Whisk until fully melted and mixture is smooth and creamy.
Pour into a mug. Top with whipped cream and a light dusting of cinnamon powder for garnish.