Preheat oven to 180°C bake. Grease and line a loaf pan (approximately 22-24cm x 13cm).
Beat butter, Chelsea Caster Sugar and Chelsea 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.