Melt the butter in a medium saucepan over medium heat. Add the packed dark brown sugar and salt, then cook for 2 to 3 minutes, stirring until glossy and slightly foamy.

In a separate bowl, whisk the whole milk, heavy cream, cornstarch, and egg yolks until completely smooth and no visible cornstarch remains.

Gradually whisk the cold milk mixture into the butter-and-sugar mixture, adding it slowly at first so the egg yolks warm gently without scrambling.

Cook over medium heat, whisking constantly, until the pudding thickens and reaches a gentle simmer. Once bubbles begin, continue cooking for about 1 minute so the cornstarch fully sets.

Remove the saucepan from the heat and whisk in the vanilla extract and optional bourbon or dark rum.

For an especially smooth finish, strain the hot pudding through a fine-mesh sieve. Pour it into six ramekins or one serving bowl.

Press plastic wrap directly against the surface of the pudding to prevent a skin from forming. Refrigerate for at least 2 hours before serving.