Place 200 g of milk in a small pot with a lid.
Add the seeds of a vanilla bean and the vanilla pod shell.
Infuse over the lowest heat for 30 minutes with the lid on.
Mix the remaining milk with cornstarch in a bowl until smooth.
Bring the milk in the pot to a boil and remove the vanilla pod.
Add the starch-milk mixture and bring it to a boil.
Thicken the mixture while stirring constantly with a whisk.
Remove the pot from heat and stir in the cold butter.
Transfer to a bowl and cover the surface directly with plastic wrap.
Let it cool and then place it in the refrigerator.