Combine the milk, cream, and half of the sugar in a medium sauce pan over medium-high heat, stirring occasionally until the liquid comes to a simmer.

As the liquid heats, whisk together the remaining sugar, salt, cornstarch, and egg yolks in a medium bowl until light and creamy.  

Once the milk mixture is hot, slowly whisk about 1 cup of the liquid into the egg mixture to temper the yolks. Pour the tempered egg mixture back into the saucepan with the hot milk mixture and reduce the heat to medium, continuing to cook while whisking constantly, until thickened and a few bubbled burst on the surface, about 30 seconds.  

Remove the pastry cream from the heat and whisk in the butter and vanilla, then transfer to a bowl and cover with a sheet of plastic wrap placed directly onto the surface of the pastry cream so a skin doesn't form. Refrigerate until chilled and firm, about 3-4 hours.