Soak saffron in 2~3tbsp warm milk. Set aside.
Drain basmati rice and grind to a coarse (semolina/ rawa) consistency. Mix the rice with 3~4tbsp milk and stir until smooth.
Pour the remaining milk into a heavy bottom pan and bring to a boil, lower the heat and simmer for 4~5 minutes, stirring occasionally.
Add the rice mixture slowly into the milk while stirring continuously to avoid lump fomration. Simmer until rice is cooked through and the mixture is thick, about 6~8 minutes.
Next add the sugar, almonds, pistachios and saffron milk. Stir well and cook till the sugar is melted and the phirni starts to thicken.
Turn off the heat and let it cool to room temperature. Stir in the mango puree and mix well. Chill for a couple of hours before serving. Garnish with chopped mango and more chopped nuts, if desired.