Preheat oven to 375 degrees. Coat baking sheets with vegetable cooking spray; set aside. In a large bowl, beat prune puree or prune butter, sugars, egg whites and vanilla to blend thoroughly. In a small bowl, mix flour, baking soda, and salt. Stir dry ingredients into prune mixture, mix completely. Stir in chocolate pieces. Drop tablespoonful onto prepared baking sheets; flatten slightly. Bake about 10 minutes until lightly browned around the edges. Remove to racks to cool completely.
Makes about 60 cookies.