Preheat the oven to 325°F (165°C). Boil a kettle of water for a water bath later. Arrange six ramekins or small ovenproof dishes in a deep baking dish.
In a medium saucepan, combine the heavy cream and milk. Heat over medium heat until it begins to simmer, then remove from heat.
Add the finely chopped dark chocolate to the hot cream mixture, stirring continuously until the chocolate is fully melted and the mixture is smooth.
In a separate mixing bowl, whisk the egg yolks with granulated sugar until the mixture is pale and slightly thickened.
Gradually add a small amount of the hot chocolate mixture to the egg yolks, whisking constantly. Continue adding the chocolate mixture slowly, whisking continuously until fully combined. This process is called tempering, and it prevents the eggs from curdling.
Stir in the vanilla extract and a pinch of salt.
Pour the chocolate mixture through a fine-mesh sieve into the ramekins, dividing it evenly.
Carefully pour the boiling water into the baking dish to reach halfway up the sides of the ramekins. This water bath (or bain-marie) ensures even and gentle cooking.
Place the baking dish with ramekins in the preheated oven and bake for about 25-30 minutes. The pots de crème should be set around the edges but still slightly wobbly in the center.
Carefully remove the ramekins from the water bath and allow them to cool to room temperature. Once cooled, cover each ramekin with plastic wrap and refrigerate for at least 4 hours, or until fully set.
Before serving, top with a dollop of whipped cream and sprinkle with grated chocolate or a dusting of cocoa powder, if desired.