Sprinkle 1 cup sugar evenly into a 10-inch cast-iron skillet; place over low heat, and cook, stirring constantly with a wooden spoon, until sugar melts and becomes a light golden brown. Gradually add boiling water, stirring to make a smooth syrup. Remove from heat, and carefully pour hot caramelized syrup into an ovenproof 5 1/2-cup ring mold.
Using oven mitts to grasp edges of mold, carefully tilt mold to coat sides with caramelized syrup. Set aside to cool.
Combine milk, remaining sugar, eggs, salt, and vanilla in a medium mixing bowl; beat well with a wire whisk. Slowly pour custard mixture into prepared mold, being careful not to disturb the caramelized syrup in bottom of mold.
Place mold in a 13- x 9- x 2- inch baking pan. Add warm water to a depth of 1 inch. Bake at 350° for 45 minutes or until a knife inserted in custard near edge of mold comes out clean. Transfer mold immediately to a pan of cold water; allow custard to cool completely. Remove mold from water, and chill thoroughly in refrigerator.
Carefully run a knife around edges of custard. Invert custard onto a serving platter, and unmold. Slice and serve.