Heat oven to 325°F (160°C). Arrange six 6 oz oven-safe ramekins in a deep roasting pan. Heat a kettle of water. Put chopped chocolate in a heatproof bowl.

Warm cream and milk in a saucepan over medium heat just until steaming with small bubbles at the edge; do not boil vigorously. Pour over chocolate, stand 1 minute, then stir until smooth.

Whisk yolks, sugar and salt in a separate bowl. Slowly whisk in about 1/2 cup warm chocolate mixture, then gradually add the rest. Stir in vanilla and pass through a fine sieve into a jug. Skim surface foam for a smoother top.

Divide custard among ramekins, leaving space below the rims. Put the roasting pan on the oven rack and carefully pour hot water around the ramekins to halfway up their sides, without splashing custard. Cover the pan loosely with foil.

Bake 30–40 minutes. The edges should be set and the centers should wobble gently as a unit, not ripple like liquid. Check a center with a thermometer: aim for 170–175°F (77–80°C). Remove sooner rather than letting the custards puff or boil.

Use tongs and an oven mitt to lift ramekins from the bath onto a rack. Cool about 45–60 minutes, then cover and refrigerate at least 4 hours. Serve cold in the ramekins. The 5-hour resting allowance includes cooling and chilling.