Cream the sugar and two sticks of butter, add eggs, then mix in buttermilk, bananas, and 1 teaspoon vanilla. Add flour, cocoa, oats, soda, and salt, and mix well. Fold in nuts and 1/2 cup chocolate chips. Pour into a sprayed or greased 9 X 13 cake pan. Sprinkle remaining chocolate chips over top and bake in a 350 degree oven for 30-35 minutes. For the frosting, cream one stick of butter until smooth, then add vanilla. Mix together the powdered sugar and cocoa. Add to butter, with a little milk at a time, until frosting reaches spreading consistency. Add more or less milk, as needed. Stir in walnuts and spread on cake.