Peel and dice the water chestnuts into 1 cm / 1/2-inch cubes. If using canned, drain well and pat dry first.

In a small bowl, stir the red food coloring into 3 tbsp water until fully dissolved.

Add the diced water chestnuts to the dye and stir to coat. Let sit for 15 minutes, stirring once or twice, until the pieces turn a deep ruby red.

Drain the water chestnuts in a fine-mesh sieve, shaking off excess liquid.

Toss the drained chestnuts with tapioca starch in a mixing bowl, working the starch into every side until each piece is fully coated with no wet spots.

Shake the coated pieces in the sieve to knock off extra starch. A thin, even coating cooks through better than a thick one.

Bring 2 liters of water to a boil in a 3-quart / 2.8L saucepan. Add the coated water chestnuts and boil for 5 to 7 minutes, until they float to the surface and turn glossy and translucent with an opaque red dot visible at the center.

Scoop the rubies out with a slotted spoon straight into a bowl of ice water. This stops the cooking and firms up the chewy tapioca shell. Let sit 5 minutes, then drain.

Combine coconut milk, 150 ml water, palm sugar, and salt in a 2-quart / 1.9L saucepan. Warm over low heat, stirring, for 3 to 4 minutes until the sugar fully dissolves. Do not let it boil, or the coconut milk can split. Cool to room temperature or chill.

Divide crushed ice among 4 serving bowls. Spoon the drained rubies and jackfruit strips over the ice.

Pour the chilled coconut milk mixture over each bowl and serve right away, while the rubies are cold and the ice is still crunchy.