Pour the milk into a small pot and heat up until boiling. (Watch out once it's boiled, it can spill very easily.)
Put the yolks in a small bowl.
Mix in the sugar. Whisk to combine.
Add the hot milk gradually to the yolk mixture, mix well.
In a small pot, melt the butter.
Add the flour and whisk.
Turn the heat to low. Add a ladle of the milk-yolk mixture. Mix well. Gradually add all the milk to the pot.
Keep stirring and cook until the cream doesn't have any flour taste.
Put the cream away from heat. Put a cling film on top. Make sure it touches the surface.
Leave it in the fridge to cool down completely.