Bring the milk to a boil in a heavy bottomed vessel. While the milk is simmering, drain the soaked basmati rice and grind to a fine paste by adding 1/2 cup of the boiled milk., slowly add the ground rice mixture and keep on stirring till it blends well into the milk and it starts to thicken.
Keep on stirring so that the bottom does not burn. The stirring could take anytime between 12-15 mts. Add cardamom powder and sugar and continue to stir till the sugar dissolves and it forms a nice creamy mixture with no lumps. Turn off flame and allow to come to room temperature. Add the mango puree to the kheer and mix well. pour to a serving bowl and chill garnish with chopped pistachios and mangoes.Serve chill.
