Grease a loaf tin, approximately 20cm x 10cm in size. Line it with baking paper.
Pre-heat the oven to 150 degrees on the bake setting.
Add pata kūteretere (softened butter), huka hāura (brown sugar) and iho hūperei (vanilla essence) in to a large bowl. Whip it on high for five minutes until light and fluffy.
Add a hēki at a time and beat for around 20 seconds after each addition.
Into a separate bowl, mash the panana (bananas). Add them in to the butter mixture.
Whip again for a minute or so until it is incorporated. Don't worry if it splits a little at this point, it will work itself out.
Sift in the puehu parāoa (flour), pēkana paura (baking powder) and pēkana houra (baking soda). Fold it all together.
Add the kirīmi kawa (sour cream) and wōnati (walnuts) and fold them in too. Leave the mixture to the side as you prepare the spiced sugar.