Add the coffee, sugar and hot water to a large bowl.
Whip it with a hand mixer until the mix turns to light caramel color and frothy, about 3 to 4 minutes. Start at a low speed and increase to a higher speed after the coffee and sugar are dissolved. Scrape down the sides with a rubber spatula once or twice during whipping. Or, whisk it vigorously by hand with a whisk (or spoon). This will take much longer, depending on how thick you want your whipped coffee to be.
Fill up a cup with about a 2/3 to 3/4 way up with milk along with some ice cubes, if desired, and top it with the whipped coffee. A rubber spatula works great to scrape it off from the bowl. You can use warm milk if you want.
You can top it off with broken up dalgona candy to serve if desired. See how to make it here.