Simmer the cream, vanilla bean and zest for 5-10 minutes.
While the cream is simmering, whisk together the egg yolks and 1/2 cup
granulated sugar.
Remove cream mixture from heat, add chocolate and espresso. Stir until
chocolate is dissolved. Strain the mixture; retain cream, discard solids
(vanilla bean, zest, some chocolate solids).
Slowly add the warm cream mixture to the egg yolk/sugar mix, whisking all
along to prevent the eggs from cooking.
Divide the resulting mix evenly among six or eight ramekins (depending on
size) or other small baking cups. Place the filled ramekins in a pan and
add water to the pan such that it goes half way up the ramekin. Bake in a
350F preheated oven for 30-35 minutes.
Remove baked ramekins from water pan and allow to cool briefly at room temp.
Then place in refrigerator for at least two hours.
Before serving, sprinkle a little (1-2 tsp) caster or brown sugar (regular
granulated works, too, just yields a different consistency crust) on top,
and carmelize with a torch or broiler. Be careful, it goes quick!!
A pastry chef recommended the following variation to lighten the custard base a bit.