Why did my baked custard split and weep?

Egg proteins set into a delicate mesh that traps the milk around them. Push them a few degrees further and that mesh contracts, squeezing the liquid out — which is exactly the watery pool you find under a split custard tart, and the little holes through the set that show it happened. Everything about baking a custard is designed to keep the whole dish within a narrow temperature band, and a water bath is the tool for it because water cannot exceed its boiling point. The drawing shows the bain-marie set up in section, including the foil wrap for a springform tin.

What you'll need

• A deep roasting tin
• A tea towel or kitchen paper for the tin base
• A kettle of hot water
• An oven thermometer (optional)

Step by step

1. Bake low.150–160°C for most baked custards, and check the oven's real temperature — an oven twenty degrees hot will split a custard that would have been perfect. See `check-oven-temperature`.
2. Set up the water bath properly.A folded tea towel or a sheet of kitchen paper on the base of the roasting tin, the dishes standing on it with a gap between them, then hot water poured in to about halfway up their sides. The paper stops the dishes sliding and stops base heat conducting straight into them.
3. Pour the water in once the tin is on the shelf.Never carry a full water bath across the kitchen. Slide the tin out on its shelf, pour from the kettle, slide it back. Use hot water so the bath does not have to heat from cold.
4. Wrap a springform tin in foil.Two layers of wide heavy-duty foil coming up the outside to above the eventual water line. Springform seams leak — see `cheesecake-base-soggy-or-crumbly`.
5. Temper the eggs into the milk gently.Warm — not boiling — milk poured slowly onto the beaten eggs and sugar while whisking. Boiling milk starts cooking the eggs before they reach the oven.
6. Strain and skim before baking.Through a fine sieve to remove chalazae and any bits of set egg, then skim the foam off the surface. Foam bakes into a rough, bubbly skin and the bubbles insulate patches of the custard so it cooks unevenly.
7. Take it out while the centre still wobbles.Nudge the dish. The edges should be set and the centre — a coin-sized area — should wobble as one piece, like set jelly, not slosh like liquid. It carries on cooking as it cools. This is where most custards are lost.
8. Cool gradually, then chill.Out of the water bath, on a rack, at room temperature before it goes in the fridge. Sudden chilling can crack the surface.

Tips

• A probe thermometer removes the guesswork: baked custards set at around 79–82°C in the centre and split above about 85°C.
• More yolks and fewer whites give a richer, more forgiving custard that sets at a slightly higher temperature and is less prone to weeping.
• A little cornflour — as in some crème brûlée and custard tart recipes — is a real insurance policy against splitting, at a small cost in silkiness.
• A tray of hot water on the shelf below is a reasonable substitute for a bain-marie when the dish will not fit in a roasting tin. It is not as effective, but it is much better than nothing.

Common mistakes

• Baking until the centre is firm — Carryover heat sets the middle after it comes out. A custard firm in the oven is split by the time it is cold.
• Filling the water bath before it goes in the oven — You will slop boiling water on yourself carrying it, and into the custard. Pour from the kettle with the tin already on the shelf.

If it doesn't work