In a food processor blitz some digestive biscuits till they crush down into fine crumbs. Transfer them to a bowl. Add melted butter to the crushed biscuits and combine the two making sure all the crumbs are well coated with butter.
Take your serving jars and put 2 tablespoons of the biscuit mixture in each. Pat the mixture down with a spoon or your fingers and refrigerate for 10 minutes.
To make the custard, in a small bowl combine custard powder and ¼ cup milk. Give this a good mix and set it aside.
In a saucepan over medium heat add rest of the milk and sugar. Bring this to a slight boil and then pour in the custard and milk mixture into the saucepan. Keep whisking till the mixture comes to a boil and becomes thick. Add cardamom powder and saffron to the mixture. Let it cool down. The custard thickens as it cools. You can also refrigerate it once cooled.
Have the gulab jamuns ready. You can either make at home (recipe) or use store bought. Remove the refrigerated jars with the biscuit, and fill with custard (if your jar's are small, use a piping bag). Then place 1-2 gulab jamun half's on the top.
Garnish with chopped pistachios. Optionally, decorate with edible silver leaf on top and dried rose petals
Repeat with remaining jars. Chill in refrigerator for a few hours before serving.