Heat the milk and lemon zest in a small heavy-based saucepan over low heat until lukewarm. Remove the lemon zest. Using an electric mixer, whisk the yolks and caster sugar in a heatproof bowl for 5 minutes or until pale and creamy. With the motor running, slowly add the hot milk, then whisk to combine. Sit the bowl over a bain marie, making sure the bowl does not touch the