In a bowl put flour, sodium bicarbonate and baking powder.Into another bowl put caster sugar, butter and whisk until it`s light and fluffy. While whisking add 1 egg at a time.Add the mashed bananas, vanilla essence along with the chopped walnuts and mix.Slowly put in the flour and mix again.Put the mixture into a loaf tin with butter paper at the bottom.Bake in a pre-heated oven at 170 degrees for about 1 hour.