Bring the water to a boil in a saucepan over medium-high heat.
Add the Thai tea mix, stir, reduce to a gentle simmer, and cook 3 minutes. Turn off the heat and let steep 2 minutes more, until very dark and fragrant.
Strain through a fine-mesh sieve lined with a coffee filter or tea sock into a heatproof pitcher, pressing gently on the leaves; discard the leaves.
While the tea is hot, stir in the granulated sugar until dissolved, then stir in the sweetened condensed milk until fully blended. Let cool until no longer steaming, about 15 minutes, or chill until cold.
Fill 4 tall glasses with ice.
Divide the tea evenly among the glasses, leaving about 1/2 inch at the top.
Drizzle 2 tablespoons evaporated milk over each glass to create a creamy layer. Serve immediately, stirring before drinking if desired.