In skillet, or in Instant Pot on saute mode, melt 2 tbsp butter. Add onion, 2 tsp salt, garlic, ginger, desired amount of cayenne pepper, paprika, chili powder, tumeric, cumin, cardamon, 1 tsp of garam masala, and coriander. Saute until onion is soft and spices are fragrant.
Add crushed tomatoes to onion mixture and use immersion blender to blend. Or, transfer mix to a blender or food processor to blend.
Return tomato mixture to Instant Pot or slow cooker. Add chicken and make sure chicken is covered with tomato mixture.
Slow Cooker: Cook on low 6-8 hours or high 4-5. Instant Pot: Cook on high for 10 minutes and let natural release for 10 minutes.
After chicken and sauce have cooked, cut remaining 2 tbsp of butter into cubes and stir in sauce until melted. Stir in coconut milk, remaining 1/2 tsp salt and remaining 1/2 tsp of garam masala.
Serve over rice and/or with naan. Garnish with cilantro.