Preheat oven to 350°. Line bottoms of 3 greased 9-in. round baking pans with parchment; grease parchment., Cream butter, shortening and sugar until light and fluffy, 5-7 minutes. Add vanilla and eggs, 1 at a time, beating well after each addition. In another bowl, whisk together flour, baking soda and salt; add to creamed mixture alternately with buttermilk, beating after each addition. Fold in walnuts., Transfer to prepared pans. Bake until a toothpick inserted in center comes out clean, 20-25 minutes. Cool in pans 10 minutes before removing to wire racks; remove paper. Cool completely., For frosting, beat butter and cream cheese until smooth. Beat in vanilla. Gradually beat in confectioners' sugar and enough buttermilk to reach spreading consistency., Spread 1 cup frosting between cake layers. Spread top of cake with an additional 1 cup frosting. Spread remaining frosting in a thin layer over side of cake. Top with additional walnuts and, if desired, orange slices.