Scrape and seed a vanilla pod into a saucepan, then add the milk and cream. Bring to a simmer, approximately 2 minutes. Remove from the heat.
In a medium bowl, add yolks and sugar. Whisk together until combined.
Very slowly, trickle some of the milk and cream mixture into the egg yolk mixture, whisking constantly. Do this very slow, or you risk scrambling the egg yolks.
When you've added all the milk mixture and everything is thoroughly combined, pour it all back into the saucepan and heat slowly on low heat. Cook until the mixture thickens, about 5 minutes, stirring constantly.
You will know it's ready when you dip a wooden spoon into the mixture and it coats the spoon.
When you swipe your finger across the spoon, the line stays.
Remove the pan from heat. Pour the mixture through a fine sieve into a jug.
Cover and chill for at least 1 hour before serving on top of (or alongside) desserts, such as cake.