Center a rack in the oven and preheat to 325 (F).
In a medium, heavy-bottomed sauce pan, combine 1 1/4 cups heavy cream and 3/4 cup whole milk. Cook over medium heat until the mixture is just about to boil, and immediately pour over the finely chopped chocolate. Let sit for a minute or so to allow the chocolate to melt, then whisk together until smooth.
In a small bowl, whisk together 5 large egg yolks, 2/3 cup granulated sugar, 1 1/2 teaspoons vanilla extract and 1/2 teaspoon kosher salt. Slowly stream this egg mixture into the chocolate mixture, whisking the chocolate mixture constantly to prevent the eggs from cooking and curdling.
Divide the mixture evenly between six 4-ounce ramekins. Arrange the ramekins in a baking dish and carefully add enough boiling water to come about two-thirds of the way up the sides of the ramekins. Transfer the baking dish to the preheated oven and bake for 30 minutes.
After 30 minutes, remove the baking dish from the oven and transfer to a wire cooling rack. Allow the pots de crème to sit in the hot water for 5 minutes. Use a knife to gently tap each ramekin — if they still appear very jiggly in the middle, let them sit for another 5 minutes in the water. The goal here is to have a custard that is set around the edges, but still slightly jiggly in the center. See baker's notes for more tips and tricks. After your the custards have passed the jiggle test to your liking, remove each ramekin from the water and transfer to a wire rack to allow to cool completely to room temperature. Once the ramekins have cooled, cover each with plastic wrap and transfer to the refrigerator to chill for 3 to 4 hours, preferably overnight.