Take the sago (tapioca pearls) in a dish and pour water just enough to fully cover it and let it soak for an hour.
The sago should have fluffed up or doubled in volume and not sticky and should mash easily when squished between your finger and thumb. If it is still hard, sprinkle some more water and let it sit for about 30 mins.
Take a saucepan and add the coconut milk, sugar and the kaffir lime leaves.
Place it over medium-high heat. Stir until the sugar is fully dissolved and the milk starts to boil.
Add the sago to the coconut milk and let it cook over medium heat until the pearls turn transparent and the mixture has come to a boil.
Remove the saucepan from heat as the pudding will continue to thicken as it cools.
At this point, you can remove the kaffir lime leaves and add in the lime zest.
Chill it in the fridge until ready to serve.
Serve the chilled pudding with mango on top cubes for a contrast in colour, taste and texture.