Pre-heat the oven to 170°C/340°F and prepare a loaf tin with baking paper and butter
In a bowl combine the flour, salt, baking powder, baking soda, cinnamon and cocoa powder
In a different large bowl, beat together the sugar and butter until light and fluffy. This will take 2-3 minutes
Add in the eggs and beat well, one at a time until combined well
Add in the mashed bananas and vanilla extract and beat until well combined
Add the rum and gently fold into the banana mixture
Add ¼ cup of the dry flour mixture to the banana mixture at a time and gently fold through. Once combine well, add another ¼ cup of the dry flour mixture. Continue until all the flour has been added
Now fold in the walnuts and chocolate until just combined
Pour the batter into the prepared loaf tin and gently tap to ensure the batter spread across the tin well
Use a small spoon to spread across the prepared streusel topping to cover the entire surface of the banana bread batter in the loaf tin
Place the banana bread in the oven and bake for about 40 minutes or so, until the top is beautifully golden and an inserted toothpick or skewer comes out clean-ish (remembering that this is a fairly fudgy banana bread and the butterscotch will be sticky!)
Remove the banana bread from the oven, then from the loaf tin and let it rest on a cooling rack for at least 20 minutes. Serve warm in thick slices