Preheat oven to 325°F (160°C) and place ramekins in a baking dish for a water bath.

In a medium saucepan, combine heavy cream and whole milk and heat over medium until steaming but not boiling, stirring occasionally.

Add chopped dark chocolate to the warm cream mixture and stir until completely melted and smooth.

In a mixing bowl, whisk egg yolks and granulated sugar until pale and fluffy.

Slowly pour the warm chocolate mixture into the egg yolk mixture while whisking continuously to temper the eggs.

Stir in vanilla extract until evenly combined.

Carefully pour the chocolate custard into the ramekins, filling them three-quarters full.

Fill the baking dish with hot water until it reaches halfway up the sides of the ramekins.

Bake for 30 to 35 minutes, or until the edges are set and the centers jiggle slightly.

Remove ramekins from the water bath and let cool to room temperature, then refrigerate for at least 4 hours.

Just before serving, sprinkle extra sugar over each custard and caramelize with a kitchen torch or under the broiler until the sugar forms a crisp top.