Blitz the pistachio nuts in a small food processor until finely chopped. Take care not to overwork them or they will become oily. Set aside.
Split the cardamom pods and remove the seeds. Pound the seeds to a powder in a pestle and mortar with 1 tsp of the sugar.
Heat the milk in a large wok or karahi over a medium heat and boil, stirring all the time with a metal spoon, until it reduces to a thick cream – you want the milk to reduce rapidly so it retains its creamy colour without browning, about 20-30 mins. Keep scraping down the sides of the pan to prevent scorching. The milk should have left the sides of the pan and have the consistency of scrambled eggs.
Spoon this thick paste into a bowl and leave to cool before chilling for 2-3 hrs. You should now have 225-250g of reduced milk solids, known as ‘khoya’.
Coarsely grate the khoya on a box grater and put it in a sturdy casserole or wok. Add the cardamom powder and remaining caster sugar and cook over a gentle heat, stirring all the time, until the sugar has dissolved and the mixture has thickened, about 10 mins.
Take the pan off the heat, add the chopped nuts and mix well. Line a baking tray with baking parchment and transfer the warm barfi mixture onto the paper, patting it out to a rectangle. Cover with another sheet of parchment, and gently roll the barfi into a 16 x 19cm rectangle about ½cm thick with a rolling pin. Leave to set for 3-4 hrs. If your kitchen is very warm, put the tray in the fridge for 30 mins.
Remove the top sheet of paper. Rest a sheet of the silver leaf, if using, on the barfi, silver side facing downwards. Gently rub the back of the paper with your fingers, the silver leaf should transfer onto the mixture – try not to touch the silver leaf with your fingers as it will stick to them. Or, press chopped pistachios onto the mixture while still warm, if you like.
Cut the firm barfi into 3 x 4cm diamond shapes. Will keep at room temperature in an airtight box for 3-4 days.