Take the yoghurt in a large bowl and fold in 1 tsp of chopped rose petals and thandai masala.
In another, clean bowl, whip the cream with a hand mixer for 2-3 minutes on high speed till light and fluffy. Add the powdered sugar and bet for a further 30 seconds
Gently fold the cream into the yoghurt mixture until combined.
Spoon the mousse into glass bowls and chill for 2 hours or overnight. Lightly sprinkle rose petals and thandai powder on top before serving.
To make the thandai masala
Toast the the nuts and spices (except the saffron and rose petals) for 2-3 minutes in a dry pan on low heat taking care not to discolour them
Grind the cooled nuts, spices, rose petals and saffron till you have a almost smooth powder. Take care to use your grinder in bursts to keep the jar cool and prevent the nuts from yielding their oily. The masala MUST be dry. Store in an airtight jar.