In a small bowl, whisk the cornstarch with a little bit of the milk until smooth.

In a saucepan, combine the remaining milk with the sugar.

Bring the milk and sugar mixture to a boil over medium heat, then remove from the heat.

In a separate bowl, whisk together the eggs and egg yolks until well combined.

Gradually pour about a third of the hot milk mixture into the egg mixture, whisking constantly to temper the eggs and prevent curdling.

Pour the tempered egg mixture back into the saucepan with the remaining hot milk.

Place the saucepan over medium heat and cook, constantly whisking, until the mixture comes to a boil and thickens into a smooth custard.

Remove the saucepan from the heat and whisk in the unsalted butter until melted and fully incorporated.

Stir in the vanilla extract until evenly distributed.

Pour the hot pastry cream into a large stainless steel bowl.

Cover the surface of the cream with plastic wrap, pressing it directly onto the cream to prevent a skin from forming.

Refrigerate the pastry cream until it is completely chilled and set, typically for at least 2 hours.