Take out of the oven as soon as they are well coloured. They should not be overcooked, otherwise they will be too dry.
Remarks
If you plan to eat them the following day instead of freshly-baked, I suggest you re-heat them just before serving (about 10 minutes at 100°C or 212°F).
Katia gives us a good tip in case you have no sticks of chocolate: use leftover chocolate truffles.
[Translator's note: since when do chocolate truffles ever last long enough to be leftover?].