1Melt the butter in a saute pan over medium-high heat, then add the onions, garlic, ginger, curry powder, chili powder, garam masala, cumin, and salt, and saute until the onions are tender and spices are fragrant.
2Combine the cooked onion and spice mixture, tomato sauce, tomato paste, and 3/4 cup of the heavy cream in a blender and blend until smooth.
3Place the raw chicken in a slow cooker insert and cover with the blended sauce, then cook on high.
4Remove the cooked chicken from the slow cooker and cut into bite-sized 1-inch pieces.
5Whisk the remaining 1/2 cup of heavy cream and cornstarch in a small bowl to create a slurry, then stir the slurry into the sauce in the slow cooker until it thickens.
6Return the chicken to the slow cooker, stir gently to coat, and turn off the slow cooker.
7Serve the butter chicken alongside jasmine rice and garnish with fresh cilantro.