In a mixing bowl, add 1 tablespoon instant coffee, 1 tablespoon sugar, and 1 tablespoon hot water.
Using a handheld mixer, stand mixer, milk frother, or a regular whisk, whip the mixture on high speed for 2–4 minutes, or until it becomes thick, light, and forms stiff peaks. (If whisking by hand, it may take about 400 vigorous strokes.)
Add ice cubes to a glass if you want an iced version. For a hot version, skip the ice.
Pour 3/4 to 1 cup of milk (chilled or heated, as you prefer) into the glass.
Spoon the whipped coffee mixture over the milk, spreading it evenly on top.
Serve immediately. Stir the whipped coffee into the milk before drinking for the best flavor and texture.