Preheat oven to 350 degrees F. Grease and flour 2 (9-inch) cake pans.
For the cake: In a medium bowl, stir together flour and baking soda; set aside. Using an electric mixer, cream together shortening and sugar in a bowl until light and fluffy, about 2 to 3 minutes. Add eggs 1 at a time, mixing well after each addition. Mix in mashed bananas, buttermilk, and vanilla. Add flour mixture, mix until just combined. Stir in black walnuts.
Pour into prepared pans. Bake until a toothpick inserted in the center of the cake comes out clean, about 35 minutes. Cool in pans on a cooling rack for 10 minutes. Remove from pans and cool completely.
Meanwhile, prepare frosting. Melt butter in small saucepan. Add brown sugar and 1/3 cup cream. Cook over medium-low heat until the sugar is dissolved, about 2 minutes. Remove from heat and add vanilla. Transfer to a large bowl.
Using a handheld electric mixer, beat in confectioners' sugar a little at a time until smooth. If frosting is too thick, add 1 tablespoon heavy cream at a time until consistency is right. Sandwich 2 layers of cake with frosting. Frost the outside of the cake. Press chopped black walnuts on the sides of the cake, if desired.
DO NOT substitute English walnuts for black walnuts.