Blend together sugar and cocoa in medium saucepan. Stir in 1 cup milk. Heat mixture over medium heat, stirring constantly with wire whisk until sugar and cocoa dissolve completely. Stir in Sriracha. Gradually whisk in remaining 3 cups milk. Heat, stirring occasionally, until hot but do not boil. Carefully ladle into serving cups. If desired, lightly dust marshmallows with red pepper and place in cups of hot chocolate.