Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9×13 pan. Place marshmallows in a single, even layer in the bottom of the pan.
Sift together flour, salt, baking soda, and baking powder. Set aside.
In 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.
Pour batter over marshmallows, making sure that marshmallows are covered, and that the batter reaches all sides of the pan.
Bake 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.