In a medium bowl, combine sugar, coffee, and water. Using a hand mixer or a whisk, vigorously whisk until mixture turns silky smooth and shiny, then continue whisking until it thickens and holds its shape. (If whisking by hand, it will take 8 to 12 minutes.)

Fill a glass most of the way full with milk and ice, then dollop and swirl the whipped coffee mixture on top, drink up.