Soak the basmati rice in about 4 tbsp of the water for an hour. Grind this, adding a little milk if necessary into a very smooth paste. You shouldn’t be able to feel the grittiness of the rice.
Take this paste and mix it into a cup of milk till smooth. Keep this aside. Put the remaining milk and sugar in a pan and bring it to a boil over medium heat, stirring frequently.
Add the rose syrup and mix well. Now stir the rice-milk mixture that was kept aside and add to the boiling milk, stirring well continuously to prevent lumps from forming.
Turn down the heat and keep stirring until the mixture starts boiling and thickening like a custard. Let it boil/ cook for about 5 minutes while stirring it constantly, until the rice is completely cooked.
Turn off the heat and add the cardamom, mix well. Let the Phirni cool to room temperature. Stir in occasionally in between so that a skin does not form on top.
Spoon the Phirni into dessert bowls or glasses and refrigerate till ready to serve. Before serving, garnish with slivered pistachios (or almonds if you prefer) and rose petals.