Preheat oven to 180 degrees C. Grease and flour a 20x30cm cake pan. Sift together the flour, baking powder and bicarb soda. Set aside.
In a large bowl, cream the margarine and sugar until light and fluffy. Beat in the eggs one at a time then stir in the vanilla and mashed bananas. Beat in the flour mixture alternately with the sour cream, mixing just until incorporated. Pour batter into prepared pan and sprinkle with topping.
To make topping: in a small bowl, mix together chocolate chips, chopped peanuts and brown sugar.
Bake in the preheated oven for 50 to 60 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.