Whisk the coffee, sugar and 3 tbsp boiling water in a bowl for approximately 5 mins until the mixture is thick and fluffy with stiff peaks. This is easiest done using an electric whisk but can be done by hand.
For hot coffee, heat the milk and pour into two heatproof glasses. For cold coffee, pour the cold milk into two glasses. Divide the coffee mixture in half and spoon evenly on top of the glasses. Serve and stir thoroughly before drinking.