01 - In a medium saucepan, heat whole milk and heavy cream over medium heat until just steaming, avoiding boiling.
02 - In a separate bowl, whisk egg yolks, granulated sugar, cornstarch, and fine salt until smooth and pale.
03 - Slowly pour half of the hot milk mixture into the egg mixture while whisking continuously to temper the yolks.
04 - Return the combined mixture to the saucepan with remaining milk and cook over medium-low heat, whisking constantly, until it thickens and bubbles, about 2 to 3 minutes.
05 - Remove from heat, then stir in softened butter and vanilla extract until the texture is smooth and glossy.
06 - Transfer to a clean bowl, cover surface with plastic wrap to prevent skin formation, and chill for at least 2 hours before use.