How to Make Pastry Cream (Cream Patissiere)

• 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 until it only just comes to a boil. 
• While the milk is heating, whisk the egg yolks and sugar together in a mixing bowl until pale. 
• Add the cornstarch to the egg mixture and whisk well until combined. 
• 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, until thickened, and it comes to a boil. This will take 1-2 minutes. When the mixture comes to a 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 until the butter has melted and fully incorporated into the cream.
• Transfer the cream to a heat-proof bowl and cover it with plastic wrap "to contact." Allow the cream to cool completely to room temperature.
• Refrigerate the pastry cream until needed.