This is perhaps the traditional British sauce. It's now fashionable to split a vanilla pod and incorporate the seeds into the sauce – this reduces the time it needs to infuse in the hot cream – but I can also recommend pure vanilla extract, which is a wonderful storecupboard standby. Begin by using a small knife to split a vanilla pod lengthways, and then use the end of a teaspoon to scoop out the seeds. Then place the pod and the seeds in a small saucepan, along with 1 pint (570 ml) double cream – you can modify this extravagance by using single cream or creamy milk. These last two might be better if the custard is for pouring, but for a trifle for a special occasion I recommend going the whole hog! Now place the pan over a gentle heat and heat it to just below simmering point.
While the cream is heating, whisk 6 large egg yolks, 1 level dessertspoon cornflour and 2 oz (50 g) golden caster sugar together in a medium bowl using a balloon whisk. The egg yolks will act as a thickening agent for the custard to make a smooth, thick sauce. And if you add just a small amount of cornflour there will never be any danger of it curdling, and even if it looks guilty of it, it will soon whip back to an amazing smoothness, because that tiny amount of cornflour will stabilise the eggs. So here's an end to boring whisking sessions of 20 minutes, instead it will be 2 minutes.
Next remove the vanilla pod from the hot cream – the distinctive flavour is imparted and the seeds remain in the cream.
Then, whisking the egg mixture all the time with one hand, gradually pour the hot cream into the bowl. When it's all in, immediately return the whole lot back to the saucepan using a rubber spatula.
Now back it goes on to the same gentle heat as you continue whisking until the custard is thick, smooth and creamy, which will happen as soon as it reaches simmering point. If you do overheat it and it looks grainy, don't worry; just transfer it to a jug or bowl and continue to whisk until it becomes smooth again. Pour the custard into a jug or bowl; cover the surface with clingfilm and leave to cool. To serve it warm later, remove the clingfilm and sit the bowl over a pan of barely simmering water.