Put the milk and cream in a heavy-based saucepan and heat gently to just below boiling point. Remove from the heat and stir in the vanilla extract.
Beat the egg yolks, sugar and cornflour in a heatproof bowl for a few minutes, until the sugar has dissolved and the mixture is pale and thick. Gradually add the warm milk, mixing constantly. When everything is mixed together, pour it back into the pan over low heat. Cook, stirring or whisking almost continuously, for about 4 minutes until the custard has thickened. Don’t let it get too hot or it will scramble, so take the pan off the heat once or twice if necessary. Pour into a serving jug and either serve straightaway, or cover the surface with plastic wrap (this stops a skin forming) and put it in the fridge to chill.