In a large bowl or zip top bag, toss the chicken around in the curry powder, salt, pepper, and cinnamon until completely coated.
Put the vegetable oil in a large skillet and heat over medium-high heat. Add the chicken and cook until all sides are browned.
Add the heavy cream or yogurt and stir. Turn the heat to medium-low, cover the skillet partially, and simmer for 15 minutes or so. The chicken will finish cooking through and the sauce will thicken.
Serve the chicken and sauce with rice and naan.