Position an oven rack in the lower third of the oven. Preheat the oven to 325°F. Place six 6-ounce ramekins in a roasting pan.
In a saucepan, combine the cream, milk, and chocolate over medium heat. Bring almost to a simmer; remove from the heat. Set aside, stirring occasionally, until the chocolate is completely melted.
In a large measuring cup, whisk together the egg yolks and the sugar. While whisking, add a little of the hot milk mixture to the egg mixture to combine. (This technique is called tempering; it makes the temperatures of two mixtures—one containing raw egg—more similar, so the egg won’t curdle in the presence of heat.) Add the remaining milk mixture, and whisk to combine. Whisk in the vanilla and salt.
Pour approximately 1/2 cup of the egg mixture into each ramekin. Cover each ramekin tightly with aluminum foil to prevent a skin from forming. Fill the roasting pan with enough boiling water to come halfway up the sides of the ramekins. Bake until the custards are just set in the center, 35 to 40 minutes.
Remove the pots from the water, and place on a wire rack to cool, about 30 minutes. (I usually remove the pots with tongs and leave the roasting pan of water in the oven. Turn the oven off and let the water cool until it is safe to remove the pan.)
When the pots de crème have cooled completely, refrigerate to chill thoroughly, preferably overnight. Just before serving, top with a dollop of whipped cream.