Combine sugar, paprika, salt, pepper and cayenne in a medium bowl. Add chicken and toss to coat. Cover and refrigerate for 15 minutes or overnight.
Heat oil in a large nonstick skillet over high heat. Add pepper-and-onion mix and cook, stirring occasionally, until the vegetables are soft, 5 to 7 minutes. Add the spice-rubbed chicken and cook, stirring, until no longer pink in the center, 3 to 5 minutes. Serve hot. Variation: Make Stir-Fried Spicy Beef by using 1 pound of stir-fry beef or thinly sliced top round steak instead of chicken in this dish.