Add the espresso, sugar, and boiling hot water to the bowl
Whip the mixture for about 3 to 5 minutes until it has a whipped cream consistency (note: you can use a hand mixer, but you get more control (and exercise) from the hand whisk
Pour anywhere from 3/4 to 1 cup of cold or warm milk into the glass cup
Dollop the whipped coffee mixture on top until the glass is filled (and then some!)
Notes
You can make variations with add-ins like cocoa powder or vanilla extract
If you prefer a cold Dalgona, add a few ice cubes into the milk before you top with the coffee mixture