Water baths are one of those things that you don’t need too often in baking, but when a recipe calls for them, they’re crucial in getting a good result. A water bath – if you’re not familiar with what they are – is almost exactly what it sounds like: it is the dish of water that baked goods requiring gentle heat are baked in, instead of placing them directly in the oven. To set one up, all you need to do is find two pans and make sure one fits easily inside the other, with at least an inch or so of room between the outer wall and the inner one. I like to use a large casserole-type dish for holding ramekins and cake pans, as the rectangular trays always give me plenty of room to maneuver round pans.
The water bath is often used when delicate foods – especially dairy and egg based custards, souffles and cheesecakes – are being cooked. You always want these types of dishes to maintain a smooth and even texture, but unfortunately they can curdle and overcook very easily. A water bath acts as a way to insulate whatever you are cooking. The temperature of the water cannot rise above 212F (if it does, the water simply turns to steam), so even though the temperature in the oven is higher than 212F, the sides of the pan are never exposed to something so extreme. The ambient heat from the oven is allowed to cook the center of the cake/custard/etc. without overcooking the sides.
I’ve already said that you need a pan larger than whatever you’re cooking in to make your water bath. To fill it, you have two options: add water before placing it into the oven or place it in the oven and add water. I prefer the later method, using a kettle or measuring cup to fill the outer pan, because it virtually eliminates the risk of spilling water all over the kitchen. The recipe will usually direct you as to whether you should start with hot or room temperature water. When baking is done, use pot holders to remove the inner pan/ramekins and leave the tray of water in the oven until it cools completely. It’s much easier to maneuver a pan that isn’t filled with near boiling water around the kitchen.