Preheat the oven to 300°F (150°C) and set a kettle of water to boil. Place six 6 oz (180 ml) ramekins in a 9x13 inch (23x33 cm) baking pan and set aside.

In a medium saucepan, combine the heavy cream, milk, bittersweet chocolate and unsweetened chocolate and warm over medium heat, stirring occasionally, until the chocolate melts.

Meanwhile, whisk the egg yolks, sugar and salt in a medium bowl and set aside.

Once the chocolate has melted, gradually pour the mixture into the egg mixture while whisking constantly.

Pass the custard through a fine mesh sieve and into a large glass measuring jug with a pour spout.

Divide the custard evenly among the 6 ramekins and pour the hot water from the kettle into the pan until it comes halfway up the sides of the ramekins. Cover the pan with foil, sealing it around the edges and poke a few holes in the top with a thin knife.

Very gently place the pan on a middle rack in the oven and bake for 30-35 minutes, until set on the edges and jiggly in the middle.

Remove from the pan and let cool completely before transferring to the refrigerator to set for at least 3 hours (or up to 2 days). Serve with a dollop of softly whipped cream.