1. In a shallow 2-quart dish, combine sherry, soy sauce, vinegar, plum sauce, chili paste, oil, and garlic. Add chicken and stir to coat. Let stand 30 minutes at room temperature or refrigerate up to 2 hours. Add scallions during last 10 minutes of marinating.
2. Prepare a medium-hot fire in a barbecue grill or preheat the broiler. Lightly oil grills rack or coat with nonstick vegetable spray. Thread chicken and scallions onto 8 to 10 small metal or soaked bamboo skewers.
3. Grill, turning, until chicken is white throughout but still juicy, 6 to 8 minutes.