1. In a bowl, combine all marinade ingredients, including chicken pieces. Toss to coat evenly. Cover and refrigerate for at least 30 minutes, or up to overnight for deeper flavor.
2. Heat butter in a large pot or Dutch oven over medium heat. Add the chopped onion and salt and cook until softened and translucent, about 5 minutes.
3. Add the garlic and ginger and cook for another minute, stirring constantly.
4. Add the diced tomatoes with their juices, heavy cream, water, tomato paste, garam masala, coriander, turmeric, cumin, and chili powder. Bring to a simmer and cook for 10 minutes, stirring occasionally.
5. Using an immersion blender or a regular blender (in batches if necessary), blend the sauce until smooth. This step creates a rich and creamy texture.
6. Add the marinated chicken pieces to the simmering sauce. Cover and cook for 15-20 minutes or until chicken is cooked and tender.
7. Garnish with fresh cilantro and extra cream. Serve hot with rice or naan bread.

Tips:

• For a deeper flavor, you can use whole spices (garam masala, coriander, cumin) and toast them in a dry pan for a minute before grinding them.
• For a richer and silkier sauce, use full-fat yogurt and heavy cream.
• You can adjust the spice level of the recipe by adding more or less chili powder.
• Leftovers can be stored in an airtight container in the refrigerator for up to 3 days. Reheat gently over low heat.