Fill a large bowl halfway with ice cubes and add cold water until the bowl is three quarters full. Place this near your stove where you can reach it quickly after draining the tapioca.

Bring 6 cups of water to a rolling boil in a large pot over high heat. The water should be bubbling vigorously across the entire surface before you add the tapioca.

Pour the tapioca pearls slowly into the boiling water while stirring constantly with a wooden spoon. Keep stirring for the first 2 minutes to prevent the pearls from sinking and sticking to the pot bottom. Cook for about 10-12 minutes.

After 12 minutes of boiling, fish out a few pearls with a slotted spoon and run them under cold water. Bite into one to check the texture: you want chewy throughout with no hard, chalky center. The pearls should look almost completely translucent with just a faint cloudiness in the very center.

Drain the tapioca through a fine mesh strainer and immediately plunge the strainer into your prepared ice bath. Swirl the pearls around in the ice water for 30 seconds to stop the cooking.

Lift the strainer from the ice bath and shake off excess water thoroughly. Transfer the drained tapioca to a clean bowl and set aside at room temperature while you prepare the mango mixture.

Peel the mangoes using a sharp knife or vegetable peeler. Cut the flesh away from the flat pit by slicing along each side, then trim any remaining fruit from the edges of the pit. Reserve one third of the mango in cubes for topping.

Add two thirds of the mango flesh to your blender along with the sugar and vanilla extract. Blend on high speed for 60 seconds until completely smooth with no visible chunks remaining.

Pour the cold coconut milk and evaporated milk into the blender with the mango puree. Blend on medium speed for 20 seconds just until everything combines into a uniform golden mixture.

Taste the mixture and add more sugar one tablespoon at a time if the mixture tastes too tart, blending briefly after each addition. The mixture should taste slightly sweeter than you want the final dessert.

Pour the mango mixture into a large bowl and fold in half of the cooked tapioca pearls using a rubber spatula. Cover the bowl and refrigerate for at least 30 minutes until thoroughly chilled.

Divide the chilled mango sago among four serving glasses or bowls. Top each portion with the reserved mango cubes, remaining tapioca pearls, pomelo segments if using, and fresh mint leaves.