One of the keys to this dish is the spices—Kashmiri red chili and garam masala. Kashmiri chili is known for its distinctive vibrant red color and relatively mild heat. Whereas garam masalas are a combination of spice like: cardamom pods, cinnamon sticks, whole cloves, and black peppercorns.
Using whole spices and grinding them with a spice/coffee grinder into a fine powder produces the best flavor. But if you’re just starting out, you might want to buy garam masala already made. Both spices along with the mustard oil are available at Indian markets.
Another key is marinating and grilling the chicken to give the individual pieces a charred edge and a tender center that’s delightful to bite into. But the curry or “gravy” is where the flavor is built and where a watchful eye as the sauce comes together is helpful. You don’t want to rush through these steps. Once the butter is melted and your spices are added wait for the spices to become fragrant before adding the fresh tomato and paste. Then make sure the paste becomes a bit drier/thicker, which concentrates the flavor. This part of the process will make or break the taste of your curry—so keep a watchful eye for your ingredients to cue you.