Place each piece of chicken between 2 sheets wax paper; flatten to 1/4-inch thickness using meat mallet or rolling pin.
Melt 1/4 cup plus 2 tablespoons butter in saucepan over medium heat. Add 3 tablespoons mustard; stir using wire whisk until smooth. Spread mustard mixture on both sides of chicken; coat completely with peanuts.
Melt remaining 2 tablespoons butter in large skillet over medium heat. Stir in oil. saute chicken breasts 3 to 4 minutes on each side or until golden brown. Remove chicken; drain on paper towels. Arrange chicken on serving platter; keep warm.
Combine sour cream, remaining 2 tablespoons mustard, chopped parsley, salt and pepper in small saucepan over medium heat. Cook, stirring constantly until smooth and thoroughly heated. Spoon sauce over chicken; garnish with parsley sprigs, if desired.