Put the cream and milk into a large pan
and gently bring to just below boiling
point. Meanwhile, in a large bowl, whisk
the yolks, cornflour, sugar and vanilla.
Gradually pour the hot milk mixture onto
the sugar mixture, whisking constantly.
Wipe out the saucepan and pour the
mixture back into it. Heat gently, stirring
with a wooden spoon (see Steps 1 and 2,
for stirring tips) until the custard is thickened, but
before any lumps form. Eat hot or cold.