In a medium saucepan, heat 2 cups of milk to a boiling point.
While the milk is heating, whisk egg yolks, sugar, flour, cornstarch and the remaining 1/4 cup milk.
Temper the egg mixture by slowly whisking in about 1/3 of the heated milk, then whisking in the remaining milk.
Return the mixture to the saucepan and cook over medium-high heat until the mixture thickens and boils.
Reduce heat to low and cook for another 2 minutes.
Remove from the heat and stir in the vanilla and butter pieces.
Pour into a bowl, press plastic wrap on the surface to prevent a skin from from foaming and chill in the fridge for at least 2 hours before using.