Put the instant coffee and sugar into a medium mixing bowl. Add the hot water and the vanilla extract, then stir briefly to dissolve the sugar and coffee.
Whisk the mixture until it triples in volume and becomes glossy and thick. Use a hand mixer on medium-high for 2–4 minutes, a stand mixer with the whisk attachment for 2–3 minutes, or a handheld milk frother for 3–5 minutes. If whisking by hand, beat vigorously for about 6–8 minutes.
For a slightly firmer texture, chill the whipped coffee for 3–5 minutes in the fridge to let it set while you prepare the glass.
Fill a tall glass with the ice, then pour the milk over the ice, leaving a little headroom for the foam.
Spoon or dollop the whipped coffee over the milk. Use the back of the spoon to spread it into an even layer or keep it as a cloud-like scoop on top.
Serve immediately. Stir the whipped coffee into the milk before drinking for an even, creamy flavor, or sip layered for visual impact.