3 Continue whisking for about 10 minutes, until the mixture triples in volume, froths up and becomes pale. When it reaches the desired consistency, take the container of custard out of the pot. Slightly thickened, the custard can be used as a sauce. Longer cooking will thicken the custard further, giving it the texture of mousse. Continue whisking for a minute or two to prevent the custard from sticking to its container.