Marinate chicken breast in a small bowl with chili powder, salt, pepper and turmeric. Let sit for 10 minutes or overnight in the refrigerator.
In a sauté pan over medium-high heat, brown chicken in 2 Tbsp butter, remove and set aside.
Melt another 2 Tbsp butter, add onion and bell pepper and cook until softened, about 8-10 minutes. Add ginger, garlic, garam masala, chili powder, cumin, cayenne pepper, cinnamon, salt and pepper and cook 1 minute, stirring.
Add tomato sauce and bring to a simmer. Add water, cream and dates, bringing back to a simmer. Return the chicken and continue to simmer, covered, for another 10-15 minutes.
Stir in the last 2 tbsp of butter and additional salt and pepper to taste.
Serve garnished with pistachio nuts and cilantro on top of basmati rice.