In the saucepan add milk and vanilla bean paste. If you are using real vanilla string, then cut and scrape seeds into milk.
Cook the milk over medium heat only just comes to boil.
While the milk is heating, whisk the egg yolks and sugar together in a mixing bowl untill pale.
Add the cornstarch to the egg mixture and whisk well untill combine.
Pour the hot milk in a slow and thin stream into the egg mixture, whisking constantly.
Return the mixture to the saucepan.
Cook over medium-low heat, with constant whisking, untill thickened and it comes to boil. This will take 1-2 minutes. When the mixture comes to boil, cook for about 1 minute more. This helps the cornstarch to achieve its full thickening power.
Remove the pastry cream from the heat and add the butter. Stir untill the butter has melted and fully incorporated into the cream.
Transfer the cream into heat-proof bowl and cover with plastic wrap "to contact". Allow cream to cool completely to room temperature.
Refrigerate the pastry cream untill needed.
