Method

In a medium bowl, combine the tandoori masala, ginger, garlic, and yogurt. Whisk until smooth, adjust seasonings to preference. Add the chicken and allow to marinate for at least 20 minutes and ideally for 12-24 hours, covered in the refrigerator.
Heat the butter in a heavy bottom pot over medium heat. Add the onions and sauté until they turn translucent and start to sweat, about 5-7 minutes. Add ginger and garlic and cook for 30 seconds, stirring so it doesn’t burn. Add the can of crushed tomatoes along with the chili , coriander, fenugreek and cumin and continue to cook for 5 minutes, if the mixture starts bubbling rapidly, add about ¼ cup of water and continue to cook.
Remove from heat, add the mixture to a blender and blend until smooth, you may need to add a some water to help it blend (up to ¼ cup).
At this point you can cool and refrigerate the sauce for 24-48 hours (or freeze it for up to 3 months).
Heat the oil over medium heat. Add the marinated chicken (discard any excess marinade) and cook for about 5 minutes , stirring as required to brown all sides. Add the butter chicken sauce to the pot and bring to a simmer, add the cream and garam masala and simmer for 5 minutes until the chicken is cooked through