Mahleb, a spice that comes from a sour-cherry tree that flourishes in the Middle East and southern Europe, has an almondlike sweetness that is a great foil for dark chocolate. The mahleb can be omitted if you like.
The name of this delicate dessert comes from the small lidded porcelain pots in which the desserts are traditionally baked. Be careful not to overcook them; the custard should wiggle gently when shaken.
Traditionally, a covered china cup called a "pot de creme" is used for this dessert. The cover is not used, however, while the creme is baking. You can also use small white ceramic souffle ramekins or Pyrex custard dishes.