In a saucepan, heat milk, cream, and vanilla until simmering.
In a bowl, whisk together egg yolks, sugar, and salt.
Slowly pour a third of the hot cream mixture into the egg mixture while whisking.
Return the mixture to the saucepan and cook on low heat until it thickens into custard.
Stir in butter, strain the custard, cover, and refrigerate for 1-2 hours.