Prepare all the ingredients.
In a saucepan, bring the milk to a boil.
Add the crème fraîche.
Add the caster sugar.
... and whisk the mixture until white in colour.
Add the boiling milk and crème while stirring.
Cook very slowly on a hob or on very low heat. The use of an induction hob is very useful for this kind of delicate cooking.
Constantly stir using a wooden spatula. The should coat the wooden spoon.
When finished, sieve the mixture using a or a fine sieve.
Leave to cool. Store in a cold place.