Tamarind concentrate is available as a paste or pulp, usually in a sticky, semidry block. Steep it in boiling water to extract the tart, flavorful juice; the remaining fibrous part should be discarded. You can use dark molasses and lime juice as a substitute.
Place peanuts in a food processor, and process until ground. Spoon into a bowl, and set aside.
Combine water and tamarind concentrate in a saucepan; stir well. Bring to a boil; cook 1 minute, stirring constantly. Remove from heat; let stand 20 minutes. Drain mixture in a sieve over a bowl, and discard pulp.
Heat oil in a medium saucepan over medium heat. Add shallots, lemon grass, red pepper, and garlic; saute 1 minute or until tender. Stir in ground peanuts, tamarind mixture, sugar, and salt. Bring to a boil; cook 17 minutes or until the mixture is reduced by half. Drain sauce in a sieve over a bowl; discard pulp.
Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; flatten to 1/8-inch thickness using a meat mallet. Remove plastic wrap; cut chicken into 3/4-inch-wide strips. Combine 3 tablespoons tamarind sauce and chicken strips in a heavy-duty zip-top plastic bag. Seal; marinate in refrigerator 1 hour. Remove chicken from bag; discard marinade.
Loosely thread chicken strips onto each of 8 (8-inch) skewers. Prepare the grillor broiler. Place the skewers on a grill rack or broiler pan coated with cooking spray, and cook 5 minutes on each side or until done. Serve skewers with remaining tamarind sauce and rice. Top with cucumber and cilantro; garnish with pineapple, if desired.
2 skewers, 1/4 cup sauce, 1/4 cup rice, 2 tablespoons cucumber, and 1 teaspoon cilantro.