Place the bananas in a large bowl and mash with a fork until completely puréed. Add 1/2 cup of brown sugar, the white sugar, oil, vanilla extract, and 1/2 teaspoonful of salt and whisk until combined.
Gently stir in the flour, bicarbonate of soda, baking powder, 1/2 teaspoonful of cinnamon, and the nutmeg.
Fold in the chocolate chunks and walnuts, if using.
Line a loaf tin with parchment paper and pour in the mixture.
In a small bowl, combine the remaining brown sugar, salt, and cinnamon. Sprinkle over the mixture.
Bake for 65 to 70 minutes, until a skewer inserted in the middle comes out clean.