Warm 100 g tamarind pulp in a saucepan with a splash of water over medium heat. Stir in 43.2 g jaggery, salt to taste, and the tamarind sauce spices. Simmer until the sauce reduces and thickens, then take it off the heat and let it cool completely.
Place 1000 g fish cubes in a bowl and season with 15.3 g lemon juice, 0.5 tablespoon white pepper, and salt to taste. Toss to coat evenly, cover, and chill in the refrigerator for 30 minutes.
In a wide mixing bowl, whisk 250 g hung yoghurt until smooth, then stir in 1 tablespoon red chilli paste and the cooled tamarind sauce. Lightly press the marinated fish between your palms to remove excess moisture, then toss the fish in the yoghurt mixture until thoroughly coated. Cover and refrigerate for 1 hour.
Fire up a tandoor, light a charcoal grill to medium heat, or preheat an oven grill (broiler) to medium. Thread the fish onto metal skewers, leaving about 2.5 cm (1 inch) between pieces, and spoon over any marinade left in the bowl.
Grill the skewers for 6 to 8 minutes until the fish is just cooked through.
Hang or prop the skewers over a tray for 2 to 3 minutes to drain any dripping juices. Transfer the skewers to a roasting dish, brush generously with melted butter, and return to the grill for a final 4 minutes.
Use 80% of the weight. Granulated sugar is sweeter by weight, but gives a cleaner, one-dimensional sweetness that lacks jaggery's caramel, molasses depth.
Soak and blend into a paste; use 100% to 120% of the weight. Adds rich, natural, fruity sweetness with complex, dark molasses notes that thicken the gravy.
Use 50% of the weight mixed with equal parts ground coriander and a pinch of ground cinnamon, ground cloves, and ground green cardamom. Sprinkle near the end for a warming finish.