Preheat oven to 180C/160C fan-forced. Grease a 7cm-deep, 19cm square cake pan. Line the base and sides with 2 layers of baking paper. Using an electric mixer, beat margarine, sugar and vanilla for 3 to 5 minutes or until light and fluffy. Add eggs, 1 at a time, beating until combined. Stir in sifted flour and bicarbonate of soda, yoghurt and banana until well combined. Spoon mixture into prepared pan. Level top with a spatula.
Bake for 1 hour to 1 hour 10 minutes or until a skewer inserted in the centre of the cake comes out clean (cover cake loosely with foil if over-browning during cooking). Stand in pan for 10 minutes. Turn onto a wire rack to cool.
Make Icing: Using an electric mixer, beat cream cheese and sugar together until light and fluffy. Fold in yoghurt until well combined. Spread icing over the top of the cake. Sprinkle with walnuts and top with sliced banana. Serve.