Preheat oven to 350 degrees F. Place four 6-ounce ramekins or custard cups in a 2-quart square baking dish or pan; set aside. In a small saucepan, heat milk over medium heat just until bubbles form around edge of pan. Remove from heat. Add marshmallows; stir until marshmallows are softened. In a medium bowl, combine hot milk mixture and crushed graham crackers. Add eggs, sugar, vanilla, and salt; mix well.
Divide mixture evenly among ramekins. Place baking dish on oven rack. Pour boiling water into baking dish around ramekins to a depth of 1 inch. Bake for 20 to 25 minutes or until custards are set.
Remove ramekins from water. Cool on a wire rack for 30 minutes. Serve warm.