01 - Warm the milk and heavy cream in a medium saucepan over medium heat until steaming without boiling.
02 - In a separate bowl, combine egg yolks, sugar, cornstarch, and salt; whisk until smooth and pale.
03 - Gradually add half of the hot milk mixture to the egg yolk blend, whisking continuously to temper.
04 - Return combined mixture to the saucepan with remaining milk; cook over medium-low heat, whisking constantly, until thick and bubbling, about 2 to 3 minutes.
05 - Remove from heat and stir in butter and vanilla extract until smooth and glossy.
06 - Transfer to a clean bowl, cover with plastic wrap pressed directly on the surface to prevent skin formation, and refrigerate for at least 2 hours before use.