In a microwave, melt chocolate; stir until smooth. Cool slightly. In a large bowl, beat eggs whites and sugar. Stir in prunes, vanilla and chocolate mixture. Combine flour and salt; gradually add to chocolate mixture just until moistened.
Pour into an 8-in. square baking pan coated with cooking spray. Sprinkle with walnuts. Bake at 350° for 30-32 minutes or until the top springs back when lightly touched.
Cool on a wire rack. Cut into squares; drizzle with caramel topping and dollop with whipped topping.