Although there's a bit of ice cream in this recipe, the ice makes it lighter and less rich than a typical milkshake. To keep the syrup visible on the sides of the glass, put the glass in the freezer up to 30 minutes before adding the syrup.
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.
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.