In a large bowl, beat 1/4 cup butter and sugar until crumbly, about 2 minutes. Add eggs, one at a time, beating well after each addition. Beat in applesauce and vanilla. Combine the flours, cocoa, baking soda and salt; add to butter mixture alternately with buttermilk, beating well after each addition. Transfer to two 9-in. round baking pans coated with cooking spray.
Combine the graham cracker crumbs, walnuts, chocolate chips and remaining butter. Break into small pieces; sprinkle evenly over batter.
Bake at 375° for 18-22 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
Spread whipped topping between layers and over sides of cake, leaving the crumb top exposed. Refrigerate leftovers.