In a medium saucepan, combine the milk and vanilla bean pod and seeds, then place over medium heat and bring to a simmer, about 8 minutes. Immediately remove from the heat and set aside to infuse the vanilla in the milk for 15 minutes. Remove the vanilla bean pod.

In a heat-proof bowl, whisk the egg yolks and sugar until smooth and a bit lighter in color. Sift in the cornstarch and whisk vigorously until no lumps remain.

While whisking, slowly pour in ½ cup of the hot milk mixture into the egg yolks. Then, slowly whisk in the remaining hot milk mixture. Once combined, pour the mixture through a fine mesh strainer back into the saucepan.

Cook over medium heat, gently whisking continuously, until thickened and bubbling, 5 to 8 minutes. If you are using it for a filling or piping, make sure you cook it for about a minute once it is bubbling to strengthen the set. (The target temperature is 200°F.) Remove from the heat and whisk in the butter until melted.

Transfer the pastry cream to a bowl and press plastic wrap directly onto the surface to prevent skin from forming.

Chill for at least 2 hours or until ready to serve. The custard can be made up to 24 hours in advance. Whisk the pastry cream again until smooth before using, as it will have set into a firm custard.