Heat the oven to 300°F (150°C). Arrange ramekins in a roasting pan. Warm cream and milk in a saucepan until steaming, without a hard boil.
Remove from heat and add chopped chocolate. Wait 2 minutes, then stir until the mixture is smooth and all chocolate pieces have melted.
Whisk egg yolks, sugar, vanilla, and salt gently in a bowl. Gradually whisk in the warm chocolate cream in a thin stream.
Strain the custard into a pouring jug to catch any egg strands. Divide evenly between ramekins, leaving room at the top.
Place the roasting pan on the oven rack and carefully add hot water halfway up the ramekin sides. Keep water out of the chocolate mixture.
Bake 30–40 minutes, until edges are set and centers tremble gently. A thermometer in the center should read around 170–175°F (77–79°C).
Remove ramekins carefully from the hot water and cool on a rack about 15 minutes. Refrigerate at least 4 hours until fully chilled.
Serve directly in the ramekins, optionally with a little whipped cream. Keep portions modest and return unused custards to the refrigerator promptly.
