Having said this, before you start, also fill your sink with cold water and some ice. I remember reading Nigella Lawson’s recipes for custards and she advises to do this so that, if while cooking your custard, it does begin to curdle, you run the pot over to the sink, plunge it in (without getting any water in the custard) and, with your whisk, whip the custard ferociously until the custard smoothens out or your arm falls off. The idea here is like shocking blanched green beans – stop the cooking process in its tracks. I’ve yet to have to do this but when she tells you how to make custard for an English trifle, damn it, you listen.