Set up an oven water bath by placing the baking dish inside a larger tray and pouring boiling water around it. The water keeps the temperature around the dish near 100°C / 212°F and softens the heat, so egg mixtures cook without curdling or drying at the edges. Fill the water to halfway up the dish, and fill the tray only after it is in the oven with the rack pulled out.
A water bath limits the heat. Even when the oven is at 170–180°C / 340–355°F, the water around the dish cannot exceed 100°C / 212°F; the mixture therefore cooks in a moderate environment rather than directly in the oven air. Egg mixtures need this limit because at high temperatures, egg proteins coagulate quickly, causing the mixture to curdle and develop a pockmarked texture that releases liquid. The water also increases humidity in the oven and helps prevent the surface from cracking. That is why a water bath works like a buffer that softens the heat.
The setup order matters. Put the mixture to be baked in its dish, then place the dish in the center of a deep roasting pan. Slide the pan into the oven with the rack pulled halfway out; only after the pan is in place, pour boiling water from a pitcher along the edge, not into the dish. The water only needs to come halfway up the dish. Filling the pan on the counter and carrying it is both heavy and risky because hot water could spill. The water should be boiling; cold water would unnecessarily lower the oven temperature.
Sealing is the second detail. Springform pans let water in; if using one, wrap the outside in two layers of heavy-duty foil, covering the bottom and sides. If you skip this step with a dessert such as cheesecake, the base will get wet and the dessert will fall apart. Pans made in one piece and porcelain dishes are safer. Placing a thin kitchen towel or paper under the dishes prevents them from sliding and softens direct heat from the bottom of the tray.
Check the water level during cooking. If it drops during a long bake, the water bath loses its effect; add hot water along the edge if needed, not cold water. Avoid opening the oven door often. The dessert is done when the edges have set and the center still jiggles slightly; if the jiggle in the center stops when the pan is shaken gently, the dessert is overbaked. You can let the baked dessert cool first in the water bath with the oven off and the door ajar; this helps reduce surface cracking. When opening the door to check, keep your face away from above the tray.
Removing the dishes takes more care than setting up the water bath because the tray contains boiling water. Pull the rack out; first remove the inner dish using two oven mitts, and remove the tray only after the water has cooled. Trying to carry a tray full of hot water risks burns. Water baths are not used only for desserts; they also work for mixtures such as vegetable purée, terrine, and egg soufflé. A stovetop bain-marie works on the same principle: heat passes to the water, then from the water to the dish. Pour the tray water down the sink after it has cooled.