Make the coconut sauce: In a small saucepan, whisk together the coconut milk, sugar, and salt. Stir the cornstarch with the cold water in a small cup until smooth, then whisk it into the coconut milk.
Warm the mixture over medium-low heat, stirring constantly, just until it lightly thickens and looks glossy, about 2 to 3 minutes. Do not boil. Transfer to a bowl and let cool for a few minutes.
Set out four tall glasses or dessert bowls. Divide the sweet red beans among the glasses, placing them in a distinct layer.
Add an even layer of sweet yellow mung beans, followed by an even layer of diced green pandan jelly. Keep the components mostly separate so the three colors show clearly.
Top each serving with about 1 cup crushed ice. Spoon the cooled coconut sauce over the ice and sprinkle with sesame seeds, if using.
Serve immediately with long spoons and straws. Stir each serving gently before eating so the ice, coconut sauce, and sweet layers combine.