In a large bowl, mix the yogurt, lemon juice, ginger-garlic paste, garam masala, turmeric powder, chili powder, cumin powder, coriander powder, and salt. Add the chicken pieces and coat them well. Cover and refrigerate for at least 30 minutes, or up to 4 hours (or overnight for best results).

Heat olive oil or ghee in a large skillet or Dutch oven over medium heat. Add the chopped onion and cook until softened, about 5-7 minutes.

Add the minced garlic and grated ginger; cook for another minute until fragrant.

Stir in the pumpkin puree and diced tomatoes (with their juice). Mix well to combine.

Add the heavy cream, chicken broth, butter, brown sugar (if using), garam masala, smoked paprika, cinnamon, and nutmeg. Stir well.

Bring the sauce to a simmer, then reduce heat to low. Let it simmer for 15-20 minutes, stirring occasionally, to allow the flavors to meld together.

Taste and adjust seasoning with salt and pepper.

You can cook the marinated chicken in different ways:

Pan-frying: Heat a tablespoon of olive oil in a separate skillet over medium-high heat. Add the chicken in a single layer and cook for 5-7 minutes per side, or until cooked through.

Baking: Preheat your oven to 400°F (200°C). Spread the marinated chicken on a baking sheet lined with parchment paper. Bake for 15-20 minutes, or until cooked through.

Grilling: Preheat your grill to medium-high heat. Grill the marinated chicken for 4-6 minutes per side, or until cooked thoroughly.

Make sure the internal temperature of the chicken reaches 165°F (74°C).

Once cooked, add the chicken to the simmering pumpkin butter sauce. Stir to coat the chicken evenly.

Let it simmer in the sauce for another 5-10 minutes. Serve hot over cooked basmati rice, garnished with fresh cilantro. Enjoy with warm naan bread, if desired.