Stir together flour, salt and cocoa powder. Set aside.
In large mixing bowl, beat eggs until light. Add sugar gradually, beating until fluffy. Fold in KELLOGG'S ALL-BRAN cereal, vanilla and margarine. Fold in flour mixture and walnuts. Spread evenly in 8 x 8-inch pan coated with cooking spray.
Bake at 350° F about 30 minutes or until wooden pick inserted near center comes out clean. Cool completely in pan.