A water bath is for food that doesn't need monitoring.
Water baths and double boilers are the cook’s solution to making or warming delicate foods, particularly egg-based sauces or custards, that would curdle, break, or scorch with too much heat. In a water bath, the dish is surrounded by hot or simmering water, and in a double boiler, the food is in a bowl suspended above it. In both cases, the water tempers the heat to permit gentle, even cooking.
A water bath, also called a bain marie (bahn mah-ree), consists of any heatproof dish of food that’s set directly in a larger, shallow container of hot or simmering water. In the oven, a water bath keeps baking custards, terrines, and mousses from overheating, and on the stovetop, it lets you warm or reheat any number of things without constant attention.
For ovenbound water baths, pour in enough simmering water to reach half to three-quarters of the way up the sides of the mold (so the top portion doesn’t overcook), replenishing it when necessary during baking. To avoid accidentally sloshing water into the food, I add boiling water from a kettle after I’ve put the pan in the oven. To prevent a skin from forming on a smooth custard, cover the mold or ramekins with foil.
A stovetop water bath is a common restaurant method for, say, keeping a fragile cream sauce warm or reheating a thick soup. Try it when you’re serving a large or elaborate meal and you need to keep several dishes warm while finishing the rest. Tall, cylindrical pans or canisters are the most space efficient when you have several items in a water bath.
For both types of water bath, I often lay a folded towel on the bottom of the larger pan. The towel keeps ramekins from rattling around, and it’s added insulation from the heat.
A double boiler works well for two-handed cooking.
A double boiler is the right choice for food that needs gentle cooking and simultaneous whisking, stirring, or blending with other ingredients: hollandaise or sabayons, melting chocolate, or stirred custards. A double boiler consists of two pans that fit together snugly—the top pan holds the food; the bottom holds simmering water. If you don’t own a set of double-boiler pans, improvise one by nesting a metal bowl on top of a saucepan.
Unlike a water bath, the water in a double boiler doesn’t touch the top pan or bowl. The cushion of hot air between the water and the food helps keep the temperature constant and the food from overheating. If your sauce does start to curdle or if melting chocolate threatens to scorch, simply lift off the top pan (or bowl, using a pot holder) for a minute before continuing.
Despite the name, don’t let double boilers boil. You’d be surprised how fast boiling water evaporates, leaving you with no more water, too much heat, and a burned pan. Also, if the top pan or bowl isn’t a tight fit, steam escaping from boiling water in the lower pan can interfere with your recipe.