In a large mixing bowl, combine the ¾ cup yogurt, 1 tbsp lemon juice, 4 cloves of garlic, 1 tsp ginger, 1 ½ tsp garam masala, 1 tsp ground cumin, 1 ½ tsp kashmiri red chili powder, ½ tsp turmeric, and 1 tsp salt. Whisk until smooth. Add the 2 lb chicken thigh pieces and toss well to coat every piece. Cover and refrigerate for at least 1 hour, or up to 12 hours. Longer is better — even 30 minutes works in a pinch.

When ready to cook, position an oven rack about 6 inches from the broiler and preheat to broil (500°F). Line a baking sheet with aluminum foil and place a wire rack on top. Arrange the marinated 2 lb chicken thigh pieces in a single layer on the rack. Broil for 10–12 minutes, flipping once halfway through, until the chicken is charred in spots and cooked through. Set aside — it will finish cooking in the sauce.

While the chicken broils, melt 2 tablespoons of butter with the 1 tbsp neutral oil in a large, deep skillet or Dutch oven over medium heat. Add the diced yellow onion and cook, stirring occasionally, for 10–12 minutes until deeply golden and soft. Don't rush this step — well-caramelized onions build the backbone of the sauce.

Add the 4 cloves of garlic and 1 tsp ginger to the onions and cook for 1–2 minutes until fragrant. Add the 1 tbsp tomato paste and stir it in, cooking for another 2 minutes until it deepens in color.

Add the 1 ½ tsp garam masala, 1 tsp ground cumin, 1 tsp coriander, 1 ½ tsp kashmiri red chili powder, and ½ tsp turmeric. Stir constantly for 30 seconds to bloom the spices in the fat. Pour in the 1 can of tomato, 1 tsp sugar, and 1 tsp salt. Stir to combine, bring to a simmer, then reduce heat to medium-low and cook uncovered for 15 minutes, stirring occasionally, until the sauce thickens and the oil starts to separate slightly at the edges.

Use an immersion blender to blend the sauce until completely smooth and silky. Alternatively, carefully transfer to a blender in batches and blend. Return the sauce to the pan over medium-low heat.

Pour in the 1 cup heavy cream and stir to combine. Add the broiled 2 lb chicken thigh pieces along with any resting juices from the baking sheet. Gently simmer for 8–10 minutes, stirring occasionally, until the chicken is tender and the sauce has thickened to a creamy, coating consistency.

Remove from heat and stir in the remaining 1 tablespoon of butter until melted and glossy. Taste and adjust 1 tsp salt as needed. The butter at the end gives the sauce its signature silky richness.

Ladle the butter chicken into bowls over steamed basmati rice or alongside warm naan. Drizzle with a little 1 cup heavy cream and scatter fresh cilantro on top. Serve immediately.