In a large bowl, combine the sour cream, lemon juice, garam masala, and cumin. Stir to mix well.
Cut chicken breasts into bite-size pieces and place into the marinade bowl.
Cover marinated chicken and place in refrigerator for at least 30 minutes. (Overnight would be ideal.)
Heat oil in a large skillet over medium high heat. Add marinated chicken pieces to skillet and cook until brown on both sides.
Remove cooked chicken and set aside.
In the same skillet, melt butter and add onions and garlic. Cook until onions are done and translucent in color.
Add spices and cinnamon stick to onions and cook for an additional 1-2 minutes.
Add cooked chicken, chicken broth, crushed tomatoes, and sugar to the skillet. Stir to combine and cook for 5-7 minutes.
Next, add in heavy cream, stir, and cook for an additional 4-5 minutes.
Remove from heat and remove cinnamon stick.
Serve over rice and enjoy.