First, preheat your oven to 325 degrees Fahrenheit (165 degrees Celsius).

Place the finely chopped bittersweet chocolate in a heatproof bowl.

In a saucepan, combine the heavy cream, whole milk, and granulated sugar. Heat the mixture over medium heat, stirring occasionally, until it reaches a simmer. Remove from heat just before it starts to boil.

In a separate bowl, whisk together the egg yolks, vanilla extract, and salt until well combined.

Slowly pour the hot cream mixture into the egg yolk mixture, whisking continuously to prevent the eggs from curdling.

Pour the combined mixture back into the saucepan and place it over low heat. Cook, stirring constantly, until the mixture thickens slightly and coats the back of a spoon. This should take about 5 to 7 minutes.

Remove the saucepan from the heat and pour the mixture through a fine-mesh sieve into the bowl with the chopped chocolate. Let it sit for a minute or two to allow the heat to melt the chocolate, then gently stir until the chocolate is completely melted and the mixture is smooth.

Divide the mixture among individual ramekins or small serving glasses. Place the ramekins in a baking dish and fill the dish with enough hot water to come halfway up the sides of the ramekins. This creates a water bath.

Carefully transfer the baking dish to the preheated oven. Bake for about 30 to 35 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. Then, cover them with plastic wrap and refrigerate for at least 2 hours or until chilled and set.

Just before serving, you can garnish the Pot de Crème with a dollop of whipped cream or some chocolate shavings, if desired.