Preheat oven to 375 degrees F. Coat baking sheets with vegetable cooking spray; set aside. In large bowl beat prune puree or butter, sugars, egg whites and vanilla to blend thoroughly. In small bowl mix flour, baking soda and salt. Stir dry ingredients into prune mixture; mix completely. Stir in chocolate pieces. Drop tablespoonfuls onto prepared baking sheets; flatten slightly. Bake about 10 minutes until lightly browned around edges. Remove to racks to cool completely.