Sift together first 5 ingredients; set aside.
Combine egg whites and 1 cup sugar; beat at medium speed of an electric mixer 3 to 4 minutes or until mixture is fluffy. Set aside.
Combine prunes, margarine, and vanilla. Add prune mixture to egg white mixture; beat at medium speed of mixer 1 minute or until combined. Gently fold egg white mixture into dry ingredients. Pour batter into a 9-inch square pan coated with cooking spray. Bake at 325° for 25 minutes or until a wooden pick inserted in center comes out clean. Let cool completely in pan on a wire rack. Sprinkle with powdered sugar, and cut into 16 squares.