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.
skewers, 1/4 cup sauce, 1/4 cup rice, 2 tablespoons cucumber, and 1 teaspoon cilantro.
