01 - In a medium saucepan, combine milk, cocoa powder, sugar, cinnamon, nutmeg, cayenne (if using), and salt. Heat over medium heat, whisking constantly until steaming but not boiling.
02 - Add the chopped dark chocolate to the hot mixture and whisk until fully melted and the mixture is smooth.
03 - Remove the saucepan from heat and stir in the vanilla extract.
04 - In a chilled mixing bowl, beat the heavy cream, powdered sugar, and vanilla extract using a hand mixer or whisk until soft peaks form.
05 - Pour the hot chocolate into serving mugs and top each with a generous spoonful of whipped cream.
06 - Sprinkle ground cinnamon or chocolate shavings on top if desired. Serve immediately.