Warm the brewing glass and phin: pour a splash of boiling water into the glass, let the phin sit on top for a minute, then dump the water out. Spoon the condensed milk into the bottom of the glass and set the phin back on top.
Spoon the ground coffee into the phin, shake gently to level the grounds, and press the gravity insert down lightly — just enough to level, not pack.
Bloom the grounds with about 2 tablespoons of the hot water and wait 30 seconds.
Fill the chamber to the top with hot water, put the lid on, and let it drip at about one drop per second, 4 to 5 minutes total.
Lift off the phin and stir the hot coffee into the condensed milk until uniform and glossy. Taste, and add another spoonful of condensed milk if you want it sweeter.
Fill a tall glass to the brim with ice, pour the coffee mixture over it, stir once, and drink within about 10 minutes.