Whisk together yogurt, lemon juice, turmeric, garam masala and cumin in a large bowl. Season chicken pieces with kosher salt, and fold chicken into the yogurt mixture, coat entirely with the marinade. Cover, and refrigerate (the longer you can let this marinate, the better).
In a large pan over medium heat, melt the butter until it starts to foam. Add the onions, and cook, stirring frequently, until translucent. Season with salt. Add the garlic, ginger and cumin seeds, and cook until the onions start to brown.
Add the cinnamon stick, sugar, tomatoes, and jalapeno, season with salt, and cook until the chiles are soft, about 10 minutes.
Add the chicken and marinade to the pan, and cook for 5 minutes, then add the chicken stock. Bring the mixture to a boil, then lower the heat and simmer, uncovered, for approximately 40 minutes.
Meanwhile, begin your rice.
Stir in cream and tomato paste into your chicken, and simmer until the chicken is cooked through, approximately 10 to 15 minutes. Garnish with the cilantro leaves.
