Make the sweet coconut cream:
Mix the coconut cream with the flour paste in a small saucepan or brass wok, stirring rigorously to incorporate. Add the salt and pandanus leaf, if using, then bring to the boil, stirring constantly to ensure the cream does not separate. When the coconut cream has thickened, add the sugar and immediately remove the pan from the heat. Stir until the sugar has dissolved. Allow to cool before serving.
Make the sticky rice:
Rinse the rice carefully to remove any excess starch without breaking the grains. Soak it overnight, with 2-3 Thai jasmine flowers, if possible.
The next day, drain the rice, rinse and place in a metal steamer; normally the raw grains of rice cling together, so they rarely fall through the holes, but if you're feeling cautious line the steamer with some rinsed muslin (cheesecloth). Make sure the rice is not piled too high in the center, nor too widely spread. Add a pandanus leaf or two to the water in the base of the steamer, if you like, then steam the rice until tender (test some grains from the area where the mound of rice is deepest); this should take about 45 minutes to 1 hour. During this time, make sure that there is plenty of water in the steamer; if you need to top up the water level, use boiling water so as not to interrupt the steaming. When you check on the rice, wipe dry the inside of the steamer lid before replacing it.
Meanwhile, stir the sugar and salt into the coconut cream until dissolved. When the rice is cooked, remove from the steamer and place in a glass or ceramic bowl, then pour over the prepared coconut cream and stir to incorporate fully. (It is important that the rice is still piping hot, so it will more completely absorb the coconut cream and become rich and glistening.) If you like, you can plunge a knotted pandanus leaf into the rice and dot the surface with a few Thai jasmine flowers. Cover and set aside in a warm place for 15 minutes before serving. Some cooks like to swaddle the bowl in a towel to keep it warm and snug!
While the rice is settling, soak the mung beans in water for about 5 minutes then drain well. Dry-roast the mung beans over a low heat in a small, heavy-based pan or a wok, shaking often, until they are golden brown and smell nutty. Remove from the heat and, if necessary, crush coarsely using a pestle and mortar or an electric grinder.
Peel the mangoes with a sharp knife, then cut the flesh away from the central stone into cheeks. Cut each cheek crosswise into five or six slices.
Divide the rice among four bowls, then place a sliced mango cheek alongside and cover with a spoonful or two of sweetened coconut cream. Sprinkle with the mung beans and serve.
Reprinted with permission from Thai Street Food: Authentic Recipes, Vibrant Traditions by David Thompson, copyright © 2009. Published by Ten Speed Press, a division of Random House, Inc.