In a heavy-based saucepan heat together the milk, double cream and vanilla pod.
Meanwhile, whisk together the yolks and caster sugar until well-blended. Whisk in the warm amaretto.
Pour half of the hot milk mixture into the egg mixture, whisking as you do so until well blended.
Pour the egg mixture into the saucepan and mix with the remaining hot milk.
Over a very low heat, cook the mixture stirring until thickened into a smooth custard, taking care not to let the mixture curdle. Serve.