Steam potatoes, covered, 13 minutes or until tender; let cool. Lightly coat potatoes with cooking spray, and cut in half. Cut and discard a thin slice from the bottom of each potato half so they will stand up. Sprinkle potato halves with salt.
Combine milk and cheese in a bowl; stir well. Spoon mixture into a pastry bag fitted with a large star tip; pipe mixture onto potato halves and into endive leaves. Top each endive leaf with one grape half. Cover and chill, if desired. Just before serving, sprinkle potato halves with caviar.