In a large pot pour milk and half of the sugar and bring to a boil. Reduce heat to medium.
Keep the milk at low simmer. In a bowl whisk egg yolks. In a separate bowl combine the remaining sugar and cornstarch. Add egg yolks to the sugar and cornstarch mixture and whisk to combine.
DO NOT MIX THE sugar, egg yolks and cornstarch too far in advance, do it when the milk is starting to boil and you are ready to add them to it.
Add 1 cup from the warm milk and carefully mix, then add mixture to the remaining warm milk and heat gently (on low heat), stirring with a wooden spoon or a whisk. Keep stirring until custard thickens, or for 3-5 minutes. This custard needs to be cooked to 185 F in order for it to become thick and not turn wattery in the fridge.
Turn off heat and add butter, which will thicken the custard even more, after it cools down. Add vanilla.Stir again and remove from heat. You can strain custard to get rid of any lumps.
Transfer to a bowl and cover tightly with plastic wrap on top. Cool to room temperature.
Serve at room temperature or cold.