Preheat oven or grill to 400 degrees.
Heat a small skillet over medium heat. Add 2 T. butter and when it melts add the garlic and ginger followed by the curry powder, garam masala, and cumin. After 30 seconds or so when it smells fragrant, add the onion and 2 teaspoons sea salt. Cook for 3 minutes more.
Add skillet contents to a blender with tomato sauce and paste. Puree.
Lay out 4 large double-thick 12” squares of foil (single layer is okay if using heavy duty foil). Divide remaining 2 T. butter and place a slice in the center of each foil. Layer chicken on top of the foil, dividing equally between each foil. Sprinkle with remaining salt (⅛ t. each). Top with tomato sauce.
Seal packets, pulling the narrowest sides up together and folding twice, then folding each of the long sides twice to seal the ends. (At this point the packets can be refrigerated for up to a day.) Place packets on grill or in the oven for 25 minutes to cook.
Swirl yogurt or coconut into finished packets and garnish with cilantro leaves. Serve with rice or warm naan.