Preheat your oven to 325°F. Prepare a baking dish by placing four ramekins in it and filling the dish with hot water until it reaches halfway up the sides of the ramekins. This will create a water bath for even cooking.

In a medium saucepan, combine the heavy cream, whole milk, and chopped bittersweet chocolate. Heat over medium heat, stirring frequently, until the chocolate is melted and the mixture is smooth. Do not let it boil.

In a separate bowl, whisk together the granulated sugar, egg yolks, vanilla extract, ground cinnamon, cayenne pepper (if using), and a pinch of salt until well combined.

Gradually pour the warm chocolate mixture into the egg yolk mixture, whisking constantly to prevent the eggs from cooking.

Strain the mixture through a fine-mesh sieve into a large measuring cup or bowl to remove any lumps.

Carefully pour the chocolate mixture into the prepared ramekins, filling each about three-quarters full.

Bake in the preheated oven for 25-30 minutes, or until the edges are set but the centers still jiggle slightly.

Remove the ramekins from the water bath and let them cool to room temperature. Once cooled, refrigerate for at least 2 hours or until chilled.

Serve chilled, garnished with a sprinkle of cinnamon or a dollop of whipped cream if desired.