Sweet Coconut Tapioca Soup with Bananas (Che Chuoi)

Bring water and coconut milk to a boil in a medium saucepan. Stir in sugar, tapioca, and salt. Reduce heat to medium-low; cook 30 minutes, stirring frequently. Stir in bananas. Remove from heat; cover, and let stand 15 minutes. Serve warm, or chill 3 hours. Sprinkle with sesame seeds, if desired.