Rinse the chicken with cold water and pat dry. Trim the excess fat. Loosen the skin from the breast and drumsticks by gently pushing your fingers in between the skin and the meat, starting at the neck cavity. Remove the backbone using kitchen shears, and then place the chicken breast-side up on a cutting board. Flatten by firmly pressing the heel of your hand down over the breastbone.
Whisk together 1/3 cup of the Tandoori Spice, the yogurt, ginger and garlic in a large glass baking dish. Coat the chicken thoroughly in the marinade, and then let marinate in the refrigerator 24 hours.
Preheat the oven to 450 degrees F.
Pat the chicken dry and sprinkle all over with the remaining 2 tablespoons Tandoori Spice. Heat a large cast-iron pan over high heat, and add the ghee. Sear the chicken skin-side down until crispy and charred, 5 to 7 minutes, and then flip and place in the oven until an instant-read thermometer stuck in the thickest, meatiest part of the thigh registers between 155 and 160 degrees F, 20 minutes. Baste generously with the lemon curd every 5 minutes. Tent the chicken with foil and let rest before serving.
Heat the coriander seeds, cumin seeds, cardamom pods, cloves, peppercorns, bay leaves and cinnamon in a medium skillet over medium-high heat, stirring often, until the cumin becomes brown, 2 to 3 minutes. Grind the spices in a spice grinder or coffee mill until powder fine, and then transfer to a bowl with the annatto seed, paprika, Kashmiri powder, salt, turmeric, mace and nutmeg. Mix thoroughly and store in an airtight container up to 4 months.
You can find kashmiri powder at your local Indian supermarket. You can also substitute a mild hot chili powder instead.