Preheat oven to 350 degrees F (175 degrees C). Lightly grease two 9x5 inch loaf pans with butter and dust with coco powder. You can just use cooking spray alone as well.
In a large bowl, cream together the shortening and sugar until light and fluffy. Stir in the eggs one at a time, beating well with each addition. Stir in the mayonnaise and bananas. Stir together the flour, salt, baking powder and baking soda. Blend the flour mixture into the banana mixture; stir just enough to evenly combine. Fold in the chocolate chips and walnuts.
Bake at 350 degrees F (175 degrees C) until a toothpick inserted into the center of the loaf comes out clean, about 50 to 75 minutes. Cool loaf in the pan for 20 minutes before removing to a wire rack to cool completely.