Wash the sabudana with light hands, using about 4 to 5 washes of water.
Soak the sabudana in milk for 1.5 to 2 hours, or until it swells and softens.
Boil the milk in a pan on high heat, stirring continuously until it starts boiling.
Add the soaked sabudana to the boiled milk and cook for 5 to 7 minutes, or until the sabudana is cooked well and translucent.
Add sugar and cardamom powder to the mixture and cook for another 2 minutes.
Grind the mangoes in a blender with some milk to make a smooth puree.
Mix the mango puree with the prepared syrup base and adjust the consistency as needed by adding cold milk.
Serve the sago mango drink chilled, garnished with chopped nuts if desired.