How do I use a bain-marie?

Two quite different arrangements get the same name, and recipes rarely say which they mean. Both exist for the same reason: they put a temperature ceiling on the cooking, which is the only reliable way to stop chocolate seizing, egg yolks scrambling and custards curdling at the edges.

Safety

Step by step

1. Set up the hob version correctly.A saucepan with 3–4cm of water in it and a heatproof bowl — glass, ceramic or stainless — resting in the pan's mouth. The bowl must be wide enough to sit on the rim and seal it, and its base must be clearly above the water, not touching. Bring the water to a bare simmer and turn it down.
2. Keep the water off the bowl and the steam out of it.If the bowl touches the water it is being heated by conduction, not steam, and it will run much hotter. If the bowl is too small, steam escapes up the side and condenses into the mixture — which is exactly what seizes chocolate. A snug fit and a bare simmer are the two things that matter.
3. Use it for the four classic jobs.Melting chocolate, where any direct heat scorches it. Whisking egg-based sauces such as hollandaise and sabayon, where direct heat scrambles the yolks. Warming custards and lemon curd to thicken without curdling. And keeping a finished sauce warm without splitting it, with the heat turned right off.
4. Set up the oven version.Stand the filled dishes in a deep roasting tin, put the tin on the oven shelf, then pour hot water from a kettle into the tin until it comes halfway up the sides of the dishes. Pouring after the tray is in the oven avoids carrying a tray of hot water across the kitchen.
5. Use the oven version for anything set with egg.Baked custards, crème caramel, crème brûlée, cheesecakes, terrines and pâtés. Oven air heats the outside of a dish much faster than the middle, so the edges of a custard set and then curdle before the centre moves. Water round the dish transfers heat gently and cannot exceed boiling.
6. Stop water getting into the food.For a springform cheesecake tin, wrap the base and sides in two layers of wide heavy-duty foil, or stand the whole tin inside a larger cake tin. Water finding its way in through a springform seam is the standard failure of this method.
7. Know when it is done and take it out promptly.A baked custard is ready when the outside is set and the middle still has a slight wobble — it will finish setting as it cools. Lift the dishes out of the water rather than leaving them in it, because the water goes on cooking them.

Tips

• You do not need special equipment. Any heatproof bowl over any saucepan is a double boiler; any deep roasting tin is a water bath.
• For melting chocolate, take the pan off the heat entirely once the water is hot. The residual heat is plenty and it removes all risk of overheating.
• A folded tea towel in the base of the roasting tin stops small dishes sliding around and slightly insulates them from the tin's base.
• Keep a kettle of hot water to hand for a long oven bake — the water level drops and dishes exposed above it will set unevenly.

Common mistakes

• Letting the water boil rather than simmer — A rolling boil throws steam and heat much harder, and in the hob version it can bounce water up against the bowl. A bare simmer is all the method needs.
• Using a bowl that sits down inside the pan and touches the water — That is a pan in a pan, not a bain-marie. The bowl runs at the water temperature by conduction and hot spots form where it touches.

Questions people ask