The Best Hot Fudge Sauce in the World

unsalted butter ((1 stick))

granulated sugar ((1 slightly generous cup))

light or dark corn syrup ((1 cup))

heavy cream ((1 pint or 2 cups))

kosher salt

whole milk ((about 2/3 cup))

cocoa powder ((1/4 cup))

water ((2 Tablespoons plus 1 teaspoon water))

light rum ((2 Tablespoons))

vanilla extract or vanilla paste

excellent quality semi-sweet chocolate (, chopped (I used Ghirardelli 60%))