1. Combine milk and sugar in a small saucepan, and cook over medium heat, stirring often, 3 minutes or until sugar dissolves. Remove pan from heat, and add chocolate and cocoa powder. Let stand 5 minutes; whisk until smooth. Let cool to room temperature, about 30 minutes.
2. Combine ice cubes, milk mixture, and ice cream in a blender, and process until smooth and slushy. Pour chocolate syrup down the insides of 4 glasses; pour in iced chocolate. Garnish, if desired.