Bring 4 cups of water to a rolling boil in a medium saucepan.
Take the pan off the heat, stir in the Thai tea mix, and steep for 5 minutes. Do not steep longer, or the tea turns bitter.
Strain through a fine-mesh strainer lined with cheesecloth or a coffee filter into a heatproof pitcher. Press the leaves gently to squeeze out the brew, then discard.
While the tea is still hot, whisk in the sugar until dissolved, then whisk in the sweetened condensed milk.
Refrigerate for at least 2 hours, until thoroughly cold.
Fill tall glasses to the top with ice. Pour the chilled tea over, leaving about an inch of room, then slowly float 1 to 2 tablespoons of evaporated milk over the back of a spoon. Serve immediately.