For green salsa: Husk, core and quarter tomatillos. Peel, pit and cut avocado into 3 cm pieces. Finely chop tomatillos, tomato and garlic in processor, using on/off turns. Transfer to heavy medium saucepan. Simmer until beginning to soften, stirring occasionally, about 3 minutes. Pour mixture into blender. Add avocado, chili, cilantro and salt. Blend until smooth, stopping occasionally to scrape down sides of blender, about 3 minutes. (Can be prepared 1 day ahead and refrigerated.) Serve at room temperature or slightly chilled. Soak mesquite chips in water to cover 30 minutes and drain. Prepare barbecue grill with white-hot coals (or set gas grill on high). For marinade: Combine oil, butter, chili powder and lemon juice in small saucepan. Stir over medium heat until butter melts. Cool marinade slightly. Peel shrimp (leave tail on) and devein. Place in non aluminum bowl. Add marinade, mixing to coat. Marinate 15 minutes, turning occasionally. Oil barbecue rack. Add mesquite to coals. Thread shrimp on skewers. Grill until shrimp turn opaque, about 1.5 minutes on each side. (Can be prepared 30 minutes ahead. Wrap in foil and keep at room temperature.) Serve with green salsa. *Available at Latin American markets.