Preheat oven to 180°C bake.
Grease and line a loaf pan (approximately 22-24cm x 13cm).
Beat butter, Caster Sugar and Soft Brown Sugar together until pale and creamy.
Add eggs one at a time, beating well after each addition.
Sift in flour, baking powder and cinnamon.
Stir a couple of times to partially mix in.
In a separate smaller bowl, dissolve baking soda in milk.
Add to butter mixture with bananas and walnuts, if using.
Mix until combined.
Pour batter into prepared loaf pan.
Bake for 1 hour or until a skewer inserted in the centre comes out clean.
Leave to cool in tin for 10 minutes before turning out onto a wire rack.
