In a medium saucepan, combine the milk, cream, and vanilla. Heat the mixture until it’s steaming but not boiling. If using a vanilla bean, let it steep for 10 minutes to infuse more flavor.

In a mixing bowl, whisk together the egg yolks, sugar, and salt. If using cornstarch, add it to the bowl now.

Slowly add the hot milk mixture to the egg yolk mixture while whisking quickly to combine. This helps to temper the eggs and avoid curdling.

Return the mixture back to the saucepan. Cook over low heat, stirring constantly, until the sauce thickens enough to coat the back of a spoon, about 4 to 6 minutes. Do not let it boil.

Strain the sauce into a clean bowl to remove any lumps or vanilla bean pieces. Serve warm or chill in the refrigerator if you prefer it cold.