Heat a dry cast iron skillet over a medium flame.  Add the cumin seeds and toast until fragrant and just beginning to brown, 1 to 2 minutes, stirring with a wooden spoon.  Transfer the cumin seeds to a large heat proof mixing bowl to cool.

Make the marinade: Place the ginger, garlic, toasted cumin, salt, garam masala, pepper, and mace in a food processor and finely chop, scraping down the sides of the bowl with a rubber spatula.  Add the lemon juice and oil and puree. Add the yogurt, 2 tablespoons cilantro, and tandoori coloring, if using, to obtain a bright orange color (this is optional, but it’s one way to give tandoori food that authentic look).  Pulse the processor in short bursts, just to mix. Transfer the marinade to a large nonreactive mixing bowl.

Stir the shrimp into the marinade, cover, and refrigerate for 1 hour.

Preheat the Hōmdoor to 500 degrees F.  Drain and skewer the shrimp. 

Place the shrimp in the Hōmdoor and cook until sizzling and browned on the outside, and cooked through, about 4 minutes. Baste with ghee or melted butter while cooking and one final time just before serving.  Sprinkle the shrimp with the remaining 2 tablespoons chopped cilantro and serve at once.