In a large mixing bowl, combine the ¾ cup plain full-fat yogurt, 1 tbsp lemon juice freshly squeezed, 4 cloves garlic minced, 1 tsp fresh ginger finely grated, 1 ½ tsp garam masala, 1 tsp ground cumin, 1 ½ tsp Kashmiri chili powder, ½ tsp turmeric, and 1 tsp kosher salt. Whisk until smooth. Add the 2 lb boneless skinless chicken thighs cut into 2-inch pieces 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 boneless skinless chicken thighs cut into 2-inch pieces 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 unsalted butter with the 1 tbsp neutral oil in a large, deep skillet or Dutch oven over medium heat. Add the diced yellow onion finely diced 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 garlic minced and 1 tsp fresh ginger finely grated 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 ground coriander, 1 ½ tsp Kashmiri chili powder, and ½ tsp turmeric. Stir constantly for 30 seconds to bloom the spices in the fat. Pour in the 1 can crushed tomatoes, 1 tsp sugar to balance acidity, and 1 tsp kosher 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 boneless skinless chicken thighs cut into 2-inch pieces 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 unsalted butter until melted and glossy. Taste and adjust 1 tsp kosher 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 fresh cilantro roughly chopped on top. Serve immediately.