Put the sugar and coffee in a mixing jug add the hot water. Whisk until the mixture turns stiff with the consistency of whipped cream.
Warm the milk in a mug (about 30 seconds in a microwave should do it). If you use a heatproof glass mug this results in a great visual impact of the white milk at the bottom of the mug with the brown fondant layer of foam atop.
Dollop the coffee foam on top of the warm milk in the mug. Serve with a long teaspoon standing in the mug to either eat the foam or mix with the milk.