Preheat oven to 350°. Grease and flour two 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 1 teaspoon vanilla. Add flour mixture, mix until just combined. Stir in 1 cup black walnuts.
Pour into prepared pans. Bake for 35 minutes. Cool in pans on a cooling rack for 10 minutes. Remove from pans and cool completely.
Meanwhile, prepare frosting. Melt butter in saucepan. Add brown sugar and cream. Cook over medium-low heat for about 2 minutes, until the sugar is dissolved. Remove from heat and add 1 tablespoon vanilla. Transfer to a large bowl.
Using a handheld electric mixer, beat in confectioners' sugar 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 icing. Ice the outside of the cake. Surround sides of cake with 2 cups crushed black walnuts.
*Cook's note: DO NOT substitute English walnuts for black walnuts.