Combine the chili powder, salt, lemon juice, and chicken pieces in a medium bowl. Cover and refrigerate at least one hour if you have the time. Otherwise, just don’t.
Meanwhile, place the cilantro, garlic, chilly, lemon juice, and 1 Tbsp. water in the bowl of a food processor or blender. Process 20-30 seconds, or until the mixture forms a paste. Set aside.
Heat the oil in a wok or heavy bottomed skillet over medium-high heat.
Add the cumin, onions, and brown sugar to the pan and toss to coat the onions in the spices. Let cook for 6-8 minutes, or until onions are nicely browned.
Remove the chicken from its marinade and add to the pan. Toss to coat it in the onions and spices. Cook for 2-3 minutes, or until the chicken pieces have turned white on all sides.
Reduce the heat to medium, then add in the ginger, tomatoes, turmeric, and cardamom and give it a good stir. Add in the contents of the food processor or blender, stir, and let cook for 2-3 minutes.
Pour in the coconut milk and water and let the contents of the pan slowly come to a boil over medium heat. Once it has reached a boil, cover the pan and reduce the heat to low. Simmer for 10 minutes. The gravy should be reduced and thick.
After ten minutes, uncover the pan and taste the sauce. Add salt as needed.