Put 4-6 vanilla ice cream, 4-6 chocolate ice cream, few ice cubes, coffee decoction, milk and chocolate sauce into a blender jar and blend to a smooth mixture.
Pour some chocolate sauce in 4 tall serving glasses and swirl. Pour the milkshake into them. Place 1 scoop of vanilla ice cream and 1 scoop of chocolate ice cream on top in each glass.
Sprinkle some dark chocolate chips and serve immediately.