1. Place chicken breasts between 2 sheets of plastic wrap, and flatten to 1/4-inch thickness, using a rolling pin or flat side of a meat mallet. Thinly slice chicken.
2. Whisk together broth, 2 tablespoons cornstarch, hot chili sauce, sugar, and vinegar.
3. Place broccoli and 2 tablespoons water in a microwave-safe bowl. Cover with plastic wrap; fold back a small edge to allow steam to escape. Microwave at HIGH 1 minute.
4. Combine chicken and soy sauce in a medium bowl; toss with remaining 2 tablespoons cornstarch to coat.
5. Melt buttery spread in a 12-inch nonstick skillet over medium-high heat; add chicken, and cook, stirring occasionally, 3 minutes or until done. Transfer chicken to a serving platter; keep warm.
6. Bring broth mixture to a boil in skillet over high heat, stirring frequently. Reduce heat to low, and simmer, stirring occasionally, 1 minute or until thickened. Add broccoli, and toss to coat. Cook 1 minute or until thoroughly heated. Pour sauce over chicken. Serve immediately.