A bain-marie is useful for melting chocolate and butter, or to cook scrambled eggs for example. It is also used to cook delicate sauces like beurre blanc or thick liquids that risk burning if not stirred regularly.
You can buy a double-boiler from a kitchen equipment store (or better directly from Amazon, here) or set up one by yourself as it is very simple to do so.
Basically a double-boiler is made up with a two containers set:
The first container (a saucepan or a pot) is put on the hob containing a little water.
The second container — a salad bowl or another smaller saucepan with a thin base – floats on water but doesn’t touch the flame. That allows the water to heat slowly by an even heat transmission from the stove.
To ensure the stability of the second container, you can put a cookie cutter in the bottom of the first container and cover it with water. Then put the second container on top and heat the water slowly.
Hint: when a recipe calls for a bain-marie, it’s better not to avoid it and follow the recipe carefully. There is no alternative for a bain-marie so stick to the recipe and don’t try to be a hero at your own risk!
Picture by I should Be Folded Laundry