In a medium saucepan set over medium heat, combine the cream, milk, sugar and salt and heat until nearly simmering. Place the cornstarch and 1/4 cup of the rum in a small mixing bowl, whisk to blend, and whisk the slurry into the cream mixture. Cook until mixture comes to a boil and continue to cook at a gentle simmer, whisking frequently, until thickened and any chalkiness is gone, 2 to 3 minutes. Remove the sauce from the heat and stir in the remaining rum, vanilla, and butter. Serve warm.