In a stand mixer with paddle attachment or with electric hand mixer beat together shortening, sugar and eggs.
Sift together flour, salt and cocoa. Add sifted mixture and buttermilk a little at a time alternating between wet and dry ingredients. Fold in the walnuts, vanilla and soda/vinegar mixture.
Fill two 8-inch round cake pans lined with parchment paper or nonstick spray. Bake at 350 degrees for 30 minutes; cool.
Top with either a vanilla butter cream or a cream cheese frosting. Press additional toasted black walnuts into the side or sprinkle on top.