1. Preheat oven to 300 degrees F. Place a folded dish towel flat inside 13-by-9-inch baking dish (this will keep the ramekins from shifting around), and arrange four 6 ounce or six 4 ounce ramekins inside.
2. In a bowl, whisk together egg yolks, granulated sugar, cocoa powder, and salt until smooth. Gradually whisk in warm cream, 1/4 cup at a time, until mixture is warm to the touch and about half of cream has been added. Pour yolk mixture back into saucepan and whisk until incorporated. Pour warm cream mixture over chopped chocolate; let sit for 30 seconds, then whisk until chocolate is melted and mixture is smooth. Whisk in vanilla.
3. Divide mixture evenly among ramekins. Place baking pan in oven, then very carefully pour hot water into the baking dish until it comes about half-way up the side of the ramekins. Bake for 30 to 35 minutes or until centers are just set but still slightly jiggly. Let cool. Chill at least 2 hours or until ready to serve.
4. Just before serving, sprinkle each custard with an even layer of granulated sugar, then caramelize with a kitchen torch, keeping the flame moving over the surface of the custards, until sugar is golden brown and bubbly. Let sit for 1 to 2 minutes to allow sugar to harden, then serve.