Preheat oven to 350°F. Coat a 9 inch square pan with cooking spray. Sift the flour, baking powder, baking soda and salt onto a piece of waxed paper or aluminum foil. Set aside. In a large bowl, beat the oil and sugar until smooth, 1 to 2 minutes. Mix in the egg whites and vanilla. With a wooden spoon, stir in the flour mixture until just combined. Fold in the prunes, walnuts and chocolate morsels. Spread the batter in the pan. Bake 25 to 30 minutes or until a wooden pick inserted in the center comes out clean or with moist-looking crumbs. Cool 5 minutes in the pan, then turn out onto a wire rack to cool completely. Cut into 12 pieces and store in an airtight container for up to 5 days.