In a medium saucepan, combine milk, vanilla pod, and seeds and ½ the sugar. Place over medium-low heat until mixture comes to a simmer.

In a medium bowl, whisk together egg yolks, cornstarch, and remaining sugar.

Temper eggs: while whisking constantly, slowly pour ⅓ of the hot milk mixture into the egg-yolk mixture, until it has been incorporated. Pour mixture back into the saucepan, and cook over medium-high heat, whisking constantly, for roughly 3-4 minutes, until it thickens a lot.

Remove from the heat and whisk in the butter. Pass the mixture through a sieve to remove the vanilla pod and any lumps.

Cover with plastic wrap, pressing it directly onto the surface of the pastry cream to prevent skin from forming and allow it to cool down to room temperature then store in the fridge for a minimum of 4 hours or up to 3 days before serving. Use to fill cream puffs, profiteroles, or pastry tarts.