A bain-marie (pronounced "bane mah-REE") is essentially a fancy way to describe a hot water bath in the culinary world. It is commonly used for cooking delicate foods such as custards. The purpose of a bain-marie is that it creates a gentle heat around the food and results in a uniform cooking process.
There are a few culinary setups that can more or less correctly be referred to as a bain-marie. In every case, the process involves using a container of hot water to deliver indirect heat, or steam, or both to the food.
Creating a Bain-Marie
The setup for making crème brûlées is probably the most common application of a bain-marie. In this system, the uncooked custards are poured into individual ramekins (small ceramic dishes), and then these ramekins are arranged in a larger baking dish. Hot water is poured into the larger dish so that it comes to about halfway up the outsides of the ramekins. Then the whole dish is transferred to the oven and baked.
By producing steam, which heats the tops of the custards more gently than dry hot air would do, this technique helps prevent the tops of the custards from cracking.
Bain-Marie and Cheesecakes
You can use the bain-marie technique to bake cheesecakes, which, being custards, are also prone to cracking on top and benefit from moist air in the oven.
Cheesecakes are generally baked in something called a springform pan, which is a two-piece contraption that allows the base and the sides to come apart, making it easier to get the cheesecake out of the pan. The downside to immersing a springform pan in water is that it can leak, and the cheesecake can get waterlogged. Some people will try to seal up the bottom of the springform with foil, but it's not a foolproof workaround by any means.
Instead, when baking a cheesecake, you can place a pan of hot water on the lower shelf of the oven and the springform on the upper shelf. This way the steam from the hot water will still envelop the top of the custard without any chance of water seeping into the cheesecake.
Using a Double Boiler
Another setup that sometimes goes by the name of bain-marie is really a double boiler. A double boiler is a cooking tool that includes a pot of hot water simmering on the stovetop, and then a bowl or insert situated above the pot of boiling water. Usually, a double boiler is used for transmitting gentle heat, like when melting chocolate or making hollandaise sauce. You can make your own double boiler by positioning one smaller pot on top of another and filling the bottom one with water. A double boiler, which includes a pair of pots that fit together, can be purchased at most kitchen supply stores.
With hollandaise sauce, which is made by stirring melted butter into beaten egg yolks to form an emulsion, it is necessary to warm the egg yolks to help them to absorb more butter. Heating the egg yolks too much will cause the eggs to curdle, giving you scrambled eggs. Whisking them over a double-boiler (or bain-marie) warms them just enough, but since the heat is indirect, it's much harder to curdle them.