Prepare the chestnuts: rinse and pat dry the Water Chestnuts, then dice them into small, bite-size cubes (about 1/2 cm).
Make the tapioca coating: in a bowl, combine the Tapioca Starch with a few tablespoons of Water and a few drops of Red Food Coloring to form a thick, spreadable paste. Add the diced chestnuts and toss until each piece is evenly coated; if the mixture is too wet, sprinkle in a little more tapioca starch.
Cook the coated chestnuts: bring a pot of water to a rolling boil, then gently drop the coated pieces in and stir to prevent sticking. Cook until the coating turns translucent and the pieces float, about 2–3 minutes. Remove with a slotted spoon and plunge into a bowl of cold water to stop cooking, then drain well.
Make the coconut milk syrup: in a small saucepan combine the Coconut Milk with about 1/2 cup additional Water, White Sugar to taste (start with 3–4 tablespoons), a pinch of Fine Salt and a bruised Pandan Leaf. Warm gently over low heat just until the sugar dissolves and the pandan aroma infuses—do not boil hard. Remove from heat and chill until cool.
Assemble and serve: in individual bowls or glasses place a scoop of Crushed Ice, add a generous spoonful of the chilled tapioca-coated chestnuts, and ladle the cold pandan-coconut milk over the top. Stir lightly and serve immediately for the best contrast of textures.
Make-ahead tip: keep the coconut milk syrup chilled and the cooked chestnuts refrigerated separately; combine with ice just before serving so the coating stays pleasantly chewy.
