Traditionally, butter chicken is mildly spiced. If you prefer a spicier dish, reserve, mince, and add the ribs and seeds from the chile. Serve with basmati rice and/or warm naan.
Total Time: 1¼ hours
1. Melt 1 tablespoon 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 3 minutes. Add water and tomato paste and whisk until no lumps of tomato paste remain. Add sugar and ½ teaspoon salt and bring to boil. Off heat, stir in cream. Using immersion blender or blender, process until smooth, 30 to 60 seconds. Return sauce to simmer over medium heat and whisk in remaining 1 tablespoon butter. Remove saucepan from heat and cover to keep warm. (Sauce can be refrigerated for up to 4 days; gently reheat sauce before adding hot chicken.)
2. Adjust oven rack 6 inches from broiler element and heat broiler. Combine chicken, yogurt, and remaining ½ teaspoon salt in bowl and toss well to coat. Using tongs, transfer chicken to wire rack set in aluminum foil—lined rimmed baking sheet. Broil until chicken is evenly charred on both sides and registers 175 degrees, 16 to 20 minutes, flipping chicken halfway through broiling.
3. Let chicken rest for 5 minutes. While chicken rests, warm sauce over medium-low heat. Cut chicken into ¾-inch chunks and stir into sauce. Stir in 1 tablespoon cilantro and season with salt to taste. Transfer to serving dish, sprinkle with remaining 1½ teaspoons cilantro, and serve.