Cream shortening and sugar together in mixing bowl. Into a separate container, sift together flour, salt, soda, and cocoa. Add to creamed ingredients alternately with buttermilk; blend well. Stir in rolled oats and chopped nuts. Bake in a greased and floured 13x9x2-inch baking pan. Bake at 350° for 35 minutes, or until a wooden pick or cake tester inserted in center comes out clean. Cool in pan on rack. Spread with Peanut Butter Frosting, below.
Blend together the peanut butter, cream, and vanilla. Add sifted confectioners' sugar, adding more cream or confectioners' sugar if necessary for spreading consistency. Spread a thin layer of icing over cooled cake.