A double boiler, also called a bain-marie, consists of two pans, one slightly smaller than the other, where the smaller pan is formed to fit on top and partway inside of the larger pan. The food being cooked is placed in the top vessel and a liquid (usually water) is placed in the bottom vessel. The amount of liquid added to the bottom vessel is such that when the top vessel is placed over the bottom, the top pan does not come into contact with the liquid. Heat is applied to bring the liquid to a gentle simmer. The food above is heated from the steam of the simmering liquid.

The unique aspect of cooking in a double boiler is the heat source. Foods cooked in a standard saucepan on a conventional stovetop are subject to direct heat and the temperature of that heat is determined by the setting of the burner. When a double boiler is used, the heat source is indirect, allowing for slow and even heating, and the temperature of the heat that is applied to the food depends on the thermal characteristics of the liquid in the bottom vessel. When that liquid is water, which it almost always is, the maximum temperature that will be applied to the food in the upper vessel (assuming sea level) is 212 degrees Fahrenheit, the boiling point of water. Using other liquids will give different maximum temperatures.

For this reason, a double boiler is used to heat foods that are too fragile to be put directly on the stovetop and is often the cooking vessel of choice when making delicate sauces or custards that might scorch or overcook if direct heat were used. A double boiler is also a good option when melting or tempering chocolate since overheating chocolate can cause it to burn and become grainy.

With all this being said, do you need a double boiler? The easy answer is ‘No'. I use the cooking method of a double boiler often, but I don't own a double boiler and I don't recommend that you buy one either. It's just not necessary. You can easily duplicate the actions of a double boiler using a pan and the appropriate size mixing bowl. In fact, there are several advantages to this arrangement. First, you don't have an extra piece of cookware to find room for in your kitchen. Secondly, you can tailor the size of your ‘double boiler' to size of the job, changing the size of the bowl and pan as needed. And, thirdly, the rounded shape of the bowl is much easier to use in terms of stirring and whisking than the straight sides of the top vessel of a true double boiler.

I have a set of tempered glass mixing bowls that work well as the top part of my double boiler. They're the perfect shape – wider and shallower than most metal mixing bowls - and the fact that they're glass allows me to get at least some idea of how much the water below is boiling, even through the condensation. This is important since you often want the water in the lower pan to be at a gentle simmer when cooking with a double boiler.