Bain-marie
water bath
A bain-marie is a method of heating in which the vessel holding the food never touches the heat source directly but is warmed through water or its steam. This caps the temperature and makes the heating even.
The physics are simple: boiling water cannot exceed 100 °C, so the food physically cannot overheat beyond that. For proteins and chocolate, which curdle or split under local overheating, that is a decisive advantage.
There are two configurations. In the steam version a bowl sits over a pan so its base stays clear of the water — this is how chocolate is melted and Swiss meringue made. In the immersion version the mould goes into a tray of hot water in the oven — this is how cheesecake, crème brûlée and terrines are baked.
With chocolate, keep every drop of water out of the mass: even a few will cause it to thicken and lose fluidity.
Watch the water level: once it boils dry the bath stops working and the heat becomes direct, with all that follows.