Make the syrup first: combine the sugar, water and cardamom in a wide pan and bring to a boil, stirring until the sugar dissolves. Simmer for 4 to 5 minutes until it feels slightly sticky between your fingers (half-string consistency). Take off the heat, stir in the rose water, saffron and lemon juice, cover and keep warm.
For the dough, mix the milk powder, flour and baking soda in a bowl. Rub in the melted ghee with your fingertips until the mixture looks like soft breadcrumbs.
Add the warm milk one spoon at a time and the teaspoon of lemon juice, gathering the mixture into a dough with your fingertips only — never knead it. It should be soft, slightly sticky and just holding together. Cover and rest for 10 minutes; the milk powder will absorb the moisture and the dough will firm up a little.
Grease your palms with ghee and divide the dough into about 20 equal portions. Roll each one between your palms with the lightest pressure into a smooth ball with no cracks — cracks now become cracks in the fryer. If the surface feels dry, dip a fingertip in milk and smooth it over.
Heat ghee or oil in a deep kadai on low-medium heat. The fat should be medium-hot, not smoking: a tiny piece of dough dropped in should rise slowly to the surface with small bubbles around it, not sizzle violently.
Slide in the balls in batches without crowding. Keep the heat low and stir the oil gently in circles with a slotted spoon so the balls rotate and colour evenly — do not prod the balls themselves for the first minute.
Fry for 5 to 7 minutes per batch, turning gently, until the jamuns are a deep, even mahogany brown all over. They will look darker than you expect; pale jamuns stay raw inside. Drain on kitchen paper for one minute.
While the syrup is warm (not boiling), slide the fried jamuns straight in. Every ball should be submerged; press any floaters down gently.
Rest the jamuns in the syrup for at least 1 hour, ideally 2, turning them once halfway so they soak evenly. They will swell and turn glossy and tender.
Serve warm — 10 seconds in the microwave is enough — garnished with pistachios and rose petals, on their own or with vanilla ice cream or rabri.