Take 1 kg of full-fat milk and pour it into a heavy-bottomed pan. Heat the milk on medium flame until it comes to a boil.
Once the milk starts boiling, reduce the flame to low and let it simmer. Keep stirring occasionally to prevent the milk from sticking to the bottom of the pan.
Continue simmering the milk until it reduces to half its original quantity. This will take approximately 30-40 minutes.
Meanwhile, deseed 4-5 ripe custard apples and mash the pulp to make a smooth paste. Set aside.
Add 100 grams of sugar to the reduced milk and mix well. Cook for another 5-7 minutes until the sugar dissolves completely.
Now, add the custard apple pulp to the milk and mix well. Cook for another 5 minutes on low flame.
Add 1/2 teaspoon of cardamom powder and a pinch of saffron strands soaked in 1 tablespoon of warm milk. Mix well and cook for another 2-3 minutes.
Turn off the flame and let the rabdi cool down to room temperature. Once cooled, refrigerate it for at least 2 hours before serving.