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.

*Cook's Note: DO NOT substitute English walnuts for black walnuts.