Stir first 5 ingredients in bowl to blend. Pierce yams with fork. Microwave on high until tender, about 12 minutes, turning once. Cut yams lengthwise in half; scoop flesh into microwave-safe bowl. Add butter; mash. Season with salt and pepper. Meanwhile, preheat broiler. Line large rimmed baking sheet with foil; place rack on sheet. Transfer 1/2 cup tamarind glaze to large bowl. Add lamb; stir to coat. Let stand 5 to 10 minutes. Thread 5 pieces of lamb onto each of 4 skewers. Sprinkle with salt. Brush with some of remaining glaze. Broil lamb to desired doneness, occasionally brushing with glaze, about 3 minutes per side for medium-rare. Rewarm yams in microwave, about 2 minutes. Divide yams among 4 plates; top yams with lamb skewers. *Chili-garlic sauce is slightly salty, spicy, and pungent; it can be found in the Asian foods section of many supermarkets and at some specialty foods stores and Asian markets. **Tamarind concentrate, a dark, seedless paste, is available at Middle Eastern, Indian, and some Asian markets.