The coffee and metal filter used for this drink are available at trung-nguyen-online.com.
Pour sweetened condensed milk into a tall, narrow heatproof glass.
Remove lid and insert from filter. Place coffee in base of filter; tap to settle into an even layer. Place insert on top of coffee grounds and adjust to even out grounds and secure in place (do not pack). Place filter on top of glass.
Wet coffee with simmering water (it should rise just above insert); wait for grounds to absorb water, 20 seconds. Add more simmering water, filling to top of filter chamber. Top with lid. Let drip, 4 to 5 minutes. Remove filter. Fill glass with ice. Serve with a spoon (to stir) and a straw.