Whisk yolks, adding sugar gradually until pale yellow and ribbon-y.
Heat cream to a bare simmer and pull form heat.
Pour the hot cream into the yolks, stirring not whisking until incorporated.
Pour mixture through a sieve into a large mixing bowl that has the chocolate in it.
Stir until chocolate melts.
Add vanilla.
Distribute into pot de crème pots or 4 ounce ramekins.
Bake for 25 minutes at 350 in bain marie.
When custard is just set to a tender jiggle, remove, cool and refrigerate.