Add the sugar, instant coffee, and boiling water to a medium-sized mixing bowl. Note: It will seem like you should use a much smaller bowl, but the mixture increases in volume as you whisk.
Whisk until the mixture lightens and has a thick, yogurt-like consistency.
Add a generous amount of ice to a cup.
Fill the cup up about three-fourths of the way with milk.
Spoon the whipped coffee mixture onto the top, until the cup is nearly full. (The more whipped coffee you add, the richer your final drink will taste.)
Stir with a straw until the milk and whipped coffee are almost fully combined.