Peel and dice the wintermelon and place into a bowl. Sprinkle with brown sugar, mix well, and allow to sit for 1 hour.
Pour the wintermelon and all juices into a pot and bring to a boil.
Sprinkle with rock sugar or coconut sugar, reduce heat and simmer for 20-30 minutes while stirring every 5 minutes to prevent the wintermelon from burning. Once cooked the wintermelon will be translucent and the juices will have thickened.
Remove from the heat and place the wintermelon and juices into a highspeed food processor and blitz until smooth.
Strain the wintermelon pureé, set aside and allow it to cool.
Meanwhile simmer 2 cups of water, remove from the heat and add the tea bags, steep for 5 minutes. Remove the tea bags and allow the tea to come to room temperature.
While the tea is cooling bring a small pot of water to a boil, add the tapioca pearls, reduce heat and simmer for 5-6 minutes.
Strain the pearls, add to a small bowl, and stir through the brown sugar. Set aside and allow to cool.
To assemble the drink: Evenly divide the soaked tapioca pearls and syrup between four glasses.
Divide the ice between the glasses, top with the wintermelon syrup, tea, and milk.