It's built upside down: sweetened condensed milk pools at the bottom of the glass, so the first sip is nearly black before it turns sweet and creamy.
Cambodian iced coffee is strong dark-roast coffee, often brewed through a small metal drip filter, poured over ice with a generous layer of sweetened condensed milk at the bottom. Vendors sell it from roadside carts and market stalls in tall glasses, ready to stir together as you drink.