In my baked custard recipe, I gave proportions of 2 whole eggs to 300 ml cream and milk. But a pouring custard, beaten in a bowl suspended over simmering water, needs a higher proportion of egg to thicken. The process can be worrying, particularly if you are unsure whether you have got the proportions right. You warm the mixture; it shows little sign of thickening; you want to carry on warming it, but are terrified that the egg will start curdling. If the mixture seems hot and is steaming, stop: it will not get any thicker. If you want to thicken it further, beat another egg, pour a little of the mixture on top of it (whisking all the time), then a little more, and then tip the contents of this bowl into the first one. Warm it again, hoping that the magic will work this time. Stop, too, as soon as you sense a thickening: if you carry on, you are likely to find bits of curdled egg in your custard.
2 whole eggs and 2 egg yolks
150 ml double cream
100 ml milk
1 tbsp caster sugar
1 tsp vanilla essence (of course, a vanilla pod is preferable)
Heat a saucepan of water over which you can suspend a bowl (I use a pyrex one, which is not ideal, but does work). The bowl should not touch the water. When the water boils, turn it down to a gentle simmer. Or use a double saucepan if you have one.
Meanwhile, heat the milk in a small saucepan with the sugar and the vanilla. Beat the eggs in your bowl. When the milk is showing bubbles on the surface, pour it into the beaten eggs -- start with just a little milk, whisking rapidly as you pour to avoid overheating the eggs and scrambling them; then add a little more, then the rest. Place the bowl in the saucepan above the simmering water, and carry on whisking until you feel the mixture thicken. Remove the bowl from the heat right away, and keep on whisking, because the egg in contact with the hot sides of the bowl might still curdle. Serve hot or cold; if you are cooling the custard, cover the bowl immediately with clingfilm, which helps to prevent the formation of a skin.