Cook the tapioca in a saucepan of simmering water for 15 minutes or until tender and translucent. Drain and rinse under cold water, then drain again. Set aside.
Using an electric mixer, whisk the egg yolks and caster sugar in a heatproof bowl until pale and creamy. Add the lime juice and zest, then sit the bowl over a bain marie, making sure the bowl doesn’t touch