In a bowl wash rice well in several changes of cold water until water is clear. Soak rice in cold water to cover overnight.

Drain rice well in a sieve. Set sieve over a large deep saucepan of simmering water (sieve should not touch water) and steam rice, covered with a kitchen towel and a lid, 30 to 40 minutes, or until tender (check water level in pan occasionally, adding more water if necessary).

While rice is cooking, in a small saucepan bring 1 cup coconut milk to a boil with 1/3 cup sugar and salt, stirring until sugar is dissolved, and remove from heat. Keep mixture warm.

Transfer cooked rice to a bowl and stir in coconut-milk mixture. Let rice stand, covered, 30 minutes, or until coconut-milk mixture is absorbed. Rice may be prepared up to this point 2 hours ahead and kept covered at room temperature.

While rice is standing, in cleaned small pan slowly boil remaining 1/3 cup coconut milk with remaining 3 tablespoons sugar, stirring occasionally, 1 minute. Transfer sauce to a small bowl and chill until cool and thickened slightly.

To serve, mold 1/4 cup servings of sticky rice on dessert plates. Drizzle desserts with sauce and sprinkle with sesame seeds. Divide mango slices among plates.