Simmer the milk and cream in a heavy-bottomed pan over gentle heat.
Scrape vanilla seeds into the liquid and add the pod to infuse.
Separate the egg yolks from the whites.
Whisk the sugar into the egg yolks until fully incorporated.
Whisk three-quarters of the simmering liquid into the egg yolk mixture.
Return the combined mixture to the pan with the remaining liquid.
Stir continuously over low heat until the mixture thickens like double cream.
Strain the custard through a sieve to remove the pod and lumps.
Serve the custard warm or allow it to cool.