Add the yogurt, garlic, lemon juice, ginger, garam masala, turmeric, cumin, salt, and peppers in a large Ziplock bag. Gently smush the spices into the yogurt to combine, then add the chicken pieces to the bag, massaging to coat it in the spiced yogurt.
Seal the Ziplock bag and refrigerate for at least 3 hours, or overnight.
In a large skillet, melt the butter (or ghee) over medium-high heat. Working in batches, cook the coated chicken pieces in the skillet for 2-4 minutes, flipping once, until the outside of the chicken is a light golden brown.
Add the tomato sauce, cream, tomato paste, and sugar to the skillet, then stir until well combined. Reduce the heat to medium-low and simmer for 20 minutes, stirring occasionally, until thickened. Taste and adjust seasonings as desired.
Serve garnished with cilantro.
