In a medium saucepan over medium heat, combine the milk and half of the sugar. Stir gently and warm until just about to boil, but do not let it boil.

In a separate bowl, whisk together the egg yolks, remaining sugar, cornstarch, and salt until the mixture is pale yellow.

Slowly pour about a cup of the warm milk into the egg mixture, whisking continuously to temper the eggs.

Pour the tempered egg mixture back into the saucepan with the remaining warm milk. Over medium heat, stir constantly and cook until the mixture thickens and begins to bubble.

Remove from heat and stir in the butter and vanilla extract until smooth.

Transfer the pastry cream to a bowl and cover the surface with plastic wrap. Let it cool at room temperature for about 30 minutes, then refrigerate for about two hours.