To put it plain,cold coffee is just that- chilled milk+sugar+coffee+lots of ice.But many things can go wrong even when the recipe is as simple,well I am speaking from experience here;).Main thing is the coffee you use,I use instant coffee powder and it works well.When added in right proportions,filter coffee decoction can work well too.You add too much and the coffee will turn out bitter and ruin the effect,so add less first and do a taste test before adding further.
Sugar - 1 tbsp
Instant coffee powder - 1/2 tsp
Chilled milk - 1 cup
Hersheys Chocolate syrup - 1 tbsp(optional,but highly recommended)
Into a blender[or a shaker]add all the ingredients and blend/shake well till everything gets mixed and the coffee turns frothy.