In a medium saucepan, bring the water to a rolling boil, and place a heatproof 1-quart pitcher in the sink. When the water boils, add the tea and remove the pan from the heat. The tea will float to the top until you stir gently to coax it into the water. When all of the tea leaves are wet, let it steep for 3 minutes.
Pour the contents of the saucepan into the pitcher, and don't worry if the dregs are left behind. Pour the tea back and forth between the pitcher and the saucepan 7 times as it becomes darker and stronger, ending up with the tea in the saucepan. Rinse out the pitcher and strain the tea through a coffee filter back into the pitcher. Add the syrup and stir to dissolve. Cool to room temperature, cover and chill until serving time.
To serve, fill tall glasses with crushed iced. Add 3/4 cup of the chilled Thai tea per glass. Top off each glass with 3 to 4 tablespoons evaporated milk. Serve as the milk cascades over the ice and swirls into the tea.
Combine the sugar and water in a small saucepan and bring to a rolling boil over high heat. Reduce the heat to maintain an active simmer and cook until liquid has thickened and colored slightly, about 5 minutes. Remove from the heat and cool to room temperature.
Pour the cooled syrup into a jar with a tight-fitting lid, cover, and store at room temperature for 1 week, or in the refrigerator for several weeks.
Recipe courtesy of Nancie McDermott, "Real Thai: The Best of Thailand's Regional Cooking", Chronicle Books, 1992