In a medium saucepan, heat milk and cream until just below simmering.
In a bowl, whisk egg yolks, sugar, and cornstarch until smooth.
Slowly whisk warm milk into yolk mixture, then return to saucepan.
Cook over low heat, stirring constantly, until thickened (3–4 mins).
Stir in caramel sauce, butter, vanilla, and sea salt.
Pour into cups, cover with plastic wrap, and refrigerate 2 hours before serving.