Put oven rack in middle position and preheat oven to 275°F.
Scrape seeds from vanilla bean into a 2- to 3-quart heavy saucepan with tip of a paring knife, then add pod, milk, cream, and sugar and bring just to a boil, stirring until sugar is dissolved. Add chocolate and cook over moderately high heat, stirring gently with a whisk, until chocolate is melted and mixture just boils. Remove from heat.
Pour mixture into a metal bowl. Set bowl in a larger bowl of ice and cold water and cool to room temperature, stirring occasionally, about 5 minutes. Whisk in yolks, then pour through a fine-mesh sieve into a 1-quart measure, discarding pod and any other solids.
Divide mixture among ramekins. Bake in a water bath until puddings are just set around edge but centers wobble when ramekins are gently shaken, about 1 hour.
Cool puddings in water bath 1 hour, then remove from water and chill, uncovered, until cold, at least 1 hour.