Soak mung beans in water till expanded, about 1 hour. Drain thoroughly.
220 g dry split mung beans
Wash pandan leaves. Cut 4 leaves 8 cm long. Separate remaining 16 leaves into 2-3 bundles. Tie each bundle into a knot.
20 young pandan leaves
Lightly oil a plate or line with parchment paper. Spread evenly with half of beans. Place cut pandan leaves on beans, spaced evenly. Top with remaining beans. Steam over rapidly boiling water till cooked but not mushy, 10 minutes or so. Remove from heat. Discard pandan leaves. Set aside till ready to serve. Cover only after beans are cool.
Place sugar in a pot. Cook over medium-high heat, swirling to ensure even browning. When sugar is lightly coloured, reduce heat to medium-low. Continue swirling till sugar is light brown, then turn off heat. Keep swirling till sugar is medium brown. Add 6 cups water. Turn on heat and bring to a boil.
200 g sugar, 1 cup
Add knotted pandan leaves and simmer gently, covered, for 10 minutes. Discard leaves. Taste and if necessary adjust sweetness. Set aside till ready to serve.
Mix sweet potato flour with ⅓ cup water to make slurry.
⅔ cup sweet potato flour, White Swallow brand (special grade)
Separate youtiao into 2 pieces. Just before serving, toast till crisp. Cut bite-size.
1 pair pair youtiao (you char kway, Chinese crullers)
Bring pandan sweet soup to a boil. Add mung beans. Once soup returns to a gentle simmer, stir and, at the same time, drizzle with sweet potato flour slurry. Bring back to a gentle simmer. Turn off heat immediately. Do not overcook or beans would turn mushy. Stirring too much and cooking after the soup has thickened would make it watery. Serve tau suan hot, topped with youtiao just before eating.