Heat the milk and mandarin zest in a small heavy-based saucepan over low heat to simmering point, then strain through a fine-mesh sieve into a heatproof bowl.
Using an electric mixer, whisk the egg yolk and caster sugar in a heatproof bowl for 5 minutes or until pale and creamy, then add the mandarin juice and Aperol. With the motor running, slowly add the hot mil