Melt 2 tablespoons butter in medium saucepan over medium heat. Add onion, garlic, ginger, and serrano and cook, stirring frequently, until mixture is softened and onion begins to brown, 6 to 8 minutes. Add garam masala, coriander, cumin, and pepper and cook, stirring frequently, until very fragrant, about 1 minute.

Add water and tomato paste and whisk until no lumps of tomato paste remain. Add ½ teaspoon salt and bring to boil. Off heat, stir in cream and remaining 6 tablespoons butter. Transfer mixture to blender and process until smooth, 30 to 60 seconds, scraping down sides of blender jar as needed. Return sauce to now empty skillet; set aside.

Adjust oven rack 6 inches from broiler element and heat broiler. Set wire rack in aluminum foil–lined rimmed baking sheet and spray with olive oil spray. Combine chicken, yogurt, and remaining ¼ teaspoon salt in bowl and toss well to coat. Transfer chicken to prepared rack and broil until chicken is evenly charred and registers 175 degrees, 16 to 20 minutes, flipping chicken halfway through broiling. Transfer chicken to cutting board and let rest for 5 minutes.

Meanwhile, warm sauce over medium-low heat. Cut chicken into ¾-inch pieces and stir into sauce. Stir in 1 tablespoon cilantro and season with salt to taste. Sprinkle with remaining 1 tablespoon cilantro. Serve.