These pots de creme have a rich, chocolaty flavor. If you prefer sweeter custards, you can add 1 tablespoon sugar with the egg yolks in step 2.
Preheat oven to 325 degrees. Put chocolate into a medium bowl. Bring 2 1/2 cups cream to a simmer in a small saucepan. Pour over chocolate in bowl. Let stand 5 minutes; stir until smooth.
Whisk together egg yolks, vanilla, and salt in a large bowl. Whisk in chocolate mixture. Pour through a fine sieve into a large measuring cup.
Place six 6-ounce ovenproof cups or ramekins into a roasting pan. Pour chocolate mixture into cups, dividing evenly. Pour enough boiling water into pan so it comes halfway up sides of cups. Cover pan with foil. Cook until custards are barely set, about 25 minutes.
Transfer cups to a wire rack, and let cool slightly. Cover with plastic, and refrigerate 1 hour (up to overnight).
Beat the remaining cup cream until stiff peaks form. Spoon a dollop over each pot de creme before serving.