Why did my chocolate mousse not set?

Different mousse recipes set on different things, and the fix depends entirely on which. A classic French mousse sets largely on cocoa butter firming as it chills, plus whipped whites. A cream-based mousse sets on whipped cream. A gelatine mousse sets on the gel. Diagnosing without knowing which structure your recipe used is guesswork, so the first step is to look at what the recipe actually relied on.

What you'll need

• Chocolate of at least 60 per cent cocoa solids
• A thermometer (optional)

Step by step

1. Chill it for longer before deciding.Four hours is the minimum for most mousses and six is safer. A mousse that looks loose at two hours very often sets completely by evening.
2. Identify which structure the recipe used.Egg whites only, cream only, both, or gelatine. That tells you where to look. A cream-based mousse that failed is a whipping problem; an egg-white mousse that failed is usually a folding or temperature problem.
3. Check the chocolate.Milk chocolate and low-percentage dark chocolate contain much less cocoa butter and much more sugar and milk solids, so they set far more softly. A recipe written for 70 per cent chocolate made with a 40 per cent bar will not set. Use at least 60 per cent unless the recipe is specifically built for milk chocolate.
4. Check the temperature at the moment of folding.Chocolate that is too hot melts the fat in the cream and deflates whipped whites on contact. Chocolate that is too cool seizes into lumps as the cold cream hits it. Around 40–45°C — warm to the touch but not hot — is the working window.
5. Check how the cream was whipped.Soft peaks, not stiff. Over-whipped cream folded into chocolate goes grainy and holds less air; unwhipped cream provides no structure at all.
6. Check the folding.Loosen the chocolate base with a quarter of the aerated component first, then fold the rest in gently in two additions. Dumping whipped cream into a stiff chocolate base and beating it in destroys everything you whipped.
7. Look for added liquid.Coffee, liqueur, orange juice — a small quantity of water-based liquid seizes chocolate and a larger one thins the mousse. Add liquid to the chocolate while melting, not to the finished mixture.
8. Decide whether to rescue.A soft mousse can be firmed by whipping fresh cream to soft peaks and folding it through, then re-chilling. A liquid one is an excellent chocolate pouring sauce or the base of an ice cream.

Tips

• A tablespoon of alcohol or strong coffee is fine and traditional; a quarter of a cup of anything watery is not. Volume is what matters, not the ingredient.
• Aquafaba whips like egg white and makes a good vegan mousse, but it is less stable — use it immediately and expect a softer set.
• Chilling the serving glasses before filling them helps the mousse set from the outside in and holds a neater shape.
• Ganache-based mousses — chocolate and cream melted together, cooled, then whipped — are considerably more reliable than folded ones. See `make-chocolate-ganache`.

Common mistakes

• Pouring hot melted chocolate into cold whipped cream — The chocolate seizes into hard flecks on contact and the cream deflates. Both mixtures need to be near each other in temperature.
• Whipping the cream to stiff peaks for a mousse — Stiff cream folds badly into a dense base, and the extra beating breaks the fat structure. Soft peaks fold in cleanly and hold more air.

If it doesn't work