In a blender, blend to a fine paste the chocolate cookies, ice cubes, drinking chocolate powder and sugar (if using).Add the cold milk and blend again. Adding the milk at this stage prevents it from spilling out of the blender jar.For the garnish: In a microwave-safe bowl, put in about 15 grams of chocolate (one small Dairy Milk) & one teaspoon of milk. Microwave till the chocolate has melted. Alternately, you can melt the chocolate on the stove-top using a double boiler method.Use a spoon to carefully leave a trail of the melted chocolate inside the serving glasses. Pour the smoothie slowly into the glasses & top with some chocolate powder or chocolate shavings on top. Serve chilled with some cookies or any snack that you like.