Preheat oven to 350. Grease and flour two 9 inch pans.
Using a electric mixer, cream together shortening and granulated sugar in a bowl. Add eggs one at a time and mix well after each addition. Stir in mashed bananas,buttermilk and vanilla. Mix flour and baking soda and add to mixture. Mix in black walnuts. Pour into prepared pans. Bake for 35 minutes. Cool in pans 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 vanilla. using a hand held electric mixer,beat in confectioner's sugar until smooth. If frosting is too thick, add 1 tbsp heavy cream at a time until consistency is right.