In a medium saucepan, combine the whole milk, heavy cream, ½ cup of sugar, cornstarch, and salt.
Cook over medium heat, stirring occasionally, until the mixture comes to a simmer.
In a separate bowl, whisk together the egg yolks and remaining ¼ cup of sugar.
Slowly pour 1 cup of the hot milk mixture into the egg yolks, whisking constantly.
Pour the egg mixture back into the saucepan with the remaining milk mixture, and cook over medium heat, stirring constantly, until the mixture thickens.
Remove from heat and stir in the vanilla extract and butter until smooth.
Strain the pastry cream through a fine-mesh sieve into a clean bowl to remove any lumps.
Place a piece of plastic wrap directly on the surface of the pastry cream to prevent a skin from forming.
Chill the pastry cream in the refrigerator until cold and set.
