Using the tip of a paring knife, pry lotus seeds into 2 halves. Remove piths if any. Add enough boiling water to cover by 3-4 cm. Soak 3 minutes and drain. Place seeds in a pot with 3 cups freshly boiled water. Bring to a boil and simmer gently, covered, for 15 minutes.
120 g dried lotus seeds, without peel
In a separate pot, melt sugar over medium-high heat. Swirl till lightly coloured. Reduce heat to medium-low. Continue swirling till sugar turns light brown. Turn off heat. Keep swirling till colour is medium brown. Add ½ cup water. If sugar solidifies, heat to dissolve. Taste to check if sugar is burnt. If it is, discard and make another batch.
6 tbsp sugar
Add sugar solution to lotus seeds. Simmer 10 minutes. Check that seeds are almost soft. If not, give them another 5 minutes. Do not overcook. Add pandan leaves. Simmer till lotus seeds are fully soft and powdery, 5-10 minutes. Discard pandan leaves.
8 pandan leaves, washed and knotted
Adjust water level and sweetness if necessary. Increase heat to medium. Stir gently and, at the same time, drizzle with sweet potato flour mixture to thicken soup. Turn off heat when soup returns to a gentle simmer. Serve hot.