Place a fine mesh strainer over a medium bowl.
Pour the milk into a medium stainless steel or enamel-coated saucepan. Split the vanilla bean in half lengthwise with a paring knife and then use the knife to scrape the seeds from the pod. Add the seeds and pod or the vanilla bean paste to the milk. Whisk the mixture together and bring to a boil over medium heat. Cover the saucepan, remove from the heat, and allow the vanilla to infuse into the milk for 1 hour. Meanwhile, combine the sugar, egg yolks and cornstarch in a separate medium bowl and whisk together until smooth.
Return the saucepan of milk and vanilla to the stovetop and bring to a boil over medium heat. Add some of the hot milk mixture to the sugar-egg mixture, 1/4 cup at a time, until the contents of the bowl are warm to the touch. This will temper the eggs so that they will not scramble when cooking. Add the contents of the bowl to the saucepan. Use a whisk to stir the mixture over medium heat while it returns to a boil. Boil for 1-2 minutes. When the first large bubble appears, whisk for another 30-45 seconds, until the mixture is glossy, light yellow and beginning to noticeably thicken.
Remove the saucepan from the heat. Strain the thickened custard through the prepared fine mesh strainer into the clean bowl, removing the vanilla pod and ensuring that there no lumps in the final custard. Cover the plastic wrap, placing it directly on top of the custard to prevent a skin from forming, and allow it to cool to a temperature between 95°F and 104°F. Alternatively, place the bowl over an ice bath to cool.
Add the room temperature butter cubes to the custard and use a handheld blender to emulsify the butter into the custard. Place a plastic wrap directly on top of the custard and set it in the refrigerator for 2-3 hours, or overnight. Once cooled, the pastry cream can be stored in an airtight container in the fridge up to 7 days.