1. To make the coconut syrup, combine all ingredients in a medium saucepan. Cook over medium heat until simmering and all of the sugar has dissolved. Transfer to a jar or container and refrigerate until cold.
2. Drain the water chestnuts and pat them dry. Cut each water chestnut into quarters and place in a large bowl. Cover with 1 cup of water and red food coloring and let rest for 10-15 minutes, or until the water chestnuts are red. Drain and set aside.
3. Place tapioca flour in a shallow bowl. Add the red water chestnuts to the bowl with the tapioca flour and toss to coat. Shake any excess tapioca flour from the water chestnuts and set aside.
4. Meanwhile, bring a large pot of water to a boil. When the water is boiling, add the water chestnuts and cook for 2-3 minutes, until they float to the top. Use a strainer or slotted spoon to remove them from the boiling water and transfer them immediately to an ice bath to cool.
5. To serve, fill a small glass about halfway with crushed ice. Top with water chestnuts and pour about a 1/4 of coconut syrup over top. Garnish with mint.