Step #1 Combine milks & cook carefully, stirring steadily over med-heat/flame for about 20-30 mins, until as thick as double cream & lightly caramelised.
Step #2 Leave to cool in a bowl.
Step #3 Beat the yolks into the mixture.
Step #4 Place bowl over pan of simmering water.
Step #5 Whisk until thickens to a custard - about 10 min.