Place the instant coffee, granulated sugar, and 0.25 cups cold water in a cocktail shaker with a tight-fitting lid or a tall jar with a lid.
Seal the shaker and shake vigorously for 20–30 seconds, until the mixture is very foamy and thick and the sugar is dissolved.
Fill a tall glass about three-quarters full with the ice cubes.
Pour the coffee foam over the ice, scraping out all the thick foam from the shaker into the glass.
Add about 0.25 cups more cold water to the shaker, swirl to collect any remaining coffee, then pour this over the foam and ice to nearly fill the glass.
Pour the evaporated milk slowly into the glass to taste, letting it swirl through the coffee and foam.
Insert a straw, give the drink a gentle stir to loosen the foam slightly, and serve immediately while very cold.