Cream margarine and sugar. Add eggs and vanilla. Beat well. Sift flour, soda, salt, baking powder together. Add buttermilk and beat well. Add dry ingredients alternately with mashed bananas.
Divide batter into 3 cake pans which will make 3 thin layers. Bake in a 350-degree oven for 20 minutes or until done. Turn cake layers out of pans; let cool.
Mix mashed banana, 2 tablespoons margarine and lemon juice; beat well. Add powdered sugar to mixture and beat until smooth and creamy, to spreading consistency. Put between layers and on side and top of stacked cake.