In a blender, add the turmeric, 2 tbsp garam masala, garlic, jalapeño, ginger, and ½ cup water. Purée until smooth. The consistency should be between a sauce and a paste. Pour into a bowl and set aside.
Add the whole tomatoes (with the juice) to the same blender and purée until smooth. Strain through a sieve into a bowl and set aside.
In a large bowl, add the cut chicken. Add 2 tbsp of the puréed ginger sauce, yogurt, and 1 tsp of salt. Mix until the chicken is fully coated. Cover with plastic wrap and marinate in the refrigerator for 30 minutes, or up to 12 hours.
Place your oven rack 6 inches from the oven grill. Transfer the chicken pieces to a baking rack on a foil-lined baking rack. Grill for 5 to 7 minutes, keeping an eye on it. It should start to char a little, but not burn. Remove the pan and move the rack to the top position under the grill. Return the pan just under the grill and grill for another minute, until the chicken is nicely charred, but not burnt. Remove from the chicken and set aside.
In a large saucepan/skillet, melt the butter over medium heat. Add the coriander seeds and cumin seeds and cook, stirring often, for 4 minutes.
Add the paprika and onions and cook for another 6 minutes, stirring frequently.
Add the remaining sauce/paste and cook for 5 minutes, stirring frequently.
Transfer the tomatoes to the pan and cook for another couple of minutes. Pour in the cream and simmer until slightly thickened, about 6 to 8 more minutes.
Stir in the chicken and the remaining 2 tbsp garam masala, chicken, and 1½ tsp salt. Simmer for another couple of minutes.
Serve at once with cooked basmati and topped with freshly chopped coriander.
