Put the milk to boil.
After it does, set aside 4/5 cup (200 ml) of it and leave to cool.
If you don't have vanilla ampoules, use 2 powder packets.
Beat the eggs with a fork and add the cooled set aside milk.
Then add the flour and stir to obtain a smooth mixture, make sure there's no flour lumps.
Pour the sugar into the milk, add the vanilla. Turn the stove on and put the milk on it again.
Once the milk starts bubbling, pour in the egg-flour mixture while stirring nonstop.
Stir until the cream thickens, remove from the stove.
Add the butter to the cream and stir until it melts.