I recommend eating this dessert within 1 or 2 days, although it will last for a couple more (no eggs or cream here, so it is safe to keep for a few days without risks!). After a couple of days, the top of the mousse will start to dry out and harden, which is why it is nicer to eat sooner. This dessert can also be made ahead - preferably up to 24 hours in advance - and kept in the fridge until it is time to serve it. So yes, you can you keep chocolate mousse overnight!