1Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 pan. Place marshmallows in a single, even layer in the bottom of the pan.
2Sift together flour, salt, baking soda, and baking powder. Set aside.
3In a large bowl, cream together the margarine and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. By hand, stir in the flour mixture alternately with the buttermilk, mixing just until incorporated. Fold in nuts and bananas.
4Pour batter over marshmallows, making sure that marshmallows are covered, and that the batter reaches all sides of the pan.
5Bake on center rack of oven for 45 minutes. Marshmallows will rise to top and make a topping. The cake may look slightly underbaked because of the marshmallow topping. Allow the cake to cool before serving.
Footnotes
Tip
Aluminum foil can be used to keep food moist, cook it evenly, and make clean-up easier.