Using Kitchen Double Boiler
A kitchen double boiler is a cooking device that consists of a saucepan partially filled with boiling water with another bowl or pot inside of it, the contents of which are heated through contact with the boiling water. The unique aspect of cooking in a kitchen 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 kitchen 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 kitchen 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 kitchen double boiler is also a good option when melting or tempering chocolate since overheating chocolate can cause it to burn and become grainy.