Preheat the oven to 400°F. Toss the butternut squash with 1 tablespoon olive oil, salt, and pepper. Spread on a baking sheet and roast for 20–25 minutes, until tender and lightly caramelized.
Meanwhile, heat the remaining olive oil in a large skillet over medium heat. Season the chicken with salt and pepper, then sear until golden on all sides, about 6–8 minutes. Remove from the pan and set aside.
In the same pan, sauté onion until soft, about 5 minutes. Stir in garlic, ginger, and tomato paste, cooking for 1–2 minutes until fragrant. Add garam masala, cumin, coriander, chili powder, and paprika, stirring to coat the onions.
Transfer the roasted squash to a blender with the onion-spice mixture and 1 cup of chicken stock. Blend until smooth and creamy. Return the sauce to the skillet, then stir in the remaining stock, butter, and coconut milk. Simmer gently for 5 minutes.
Add the chicken back to the skillet and let it simmer in the sauce for another 10 minutes, until cooked through and the sauce is thickened. Taste and adjust seasoning as needed.
Serve hot with rice or naan, garnished with fresh cilantro.