In a saucepan, add the milk, castor sugar, and 2 tablespoons of thandai powder. Bring to a boil, stirring from time to time. Once the first boil comes, immediately reduce the heat and simmer for five minutes, stirring from time to time. Once done, pour into a large shallow dish, and allow to cool. It will be used to soak the cake rusk.
Warm up a little bit of the cream, and add the saffron. Set aside to infuse and cool.
Chop the almonds and pistachios for garnish.
Add the mascarpone cheese (not straight from the fridge; let it come to room temperature), sifted icing sugar, and the rest of the double cream to a large bowl, along with the infused saffron cream. Beat to soft peaks.
Add the remaining thandai powder and whisk to combine. Do not over-mix, or the mascarpone and cream mix will spit.
To assemble, soak both sides of the cake rusk in the cooled, milk-thandai mix and place in a serving dish. Then add a layer of thundai cream. Spread evenly, and add a little of the almond and pistachio pieces. Soak the remaining cake rusk in the milk-thandai mix, then place it over the first layer of thundai cream to create a second layer. Add the remaining thundai cream and spread evenly. Make sure the top is smooth
At this point, either garnish with the remaining almond and pistachio pieces and rose petals (if using), then refrigerate to set. Alternatively, you can chill without the garnish. You can always garnish just before serving.
Refrigerate for at least four hours, before serving. Overnight, is even better. Chilling helps the flavours infuse better, and the rusk soften.