Start by soaking the mashed bananas and walnuts in the rum until just covered. Leave to soak for 30 minutes.
Preheat the oven to 180 C / Gas 4. Grease a 900g loaf tin and line with baking parchment.
In a bowl, mix together the spelt and rye flour, add in the baking powder, bicarbonate of soda, sugar and cinnamon. Add the mashed banana, walnut and rum mix to the flour and stir in. Add one small egg, peanut butter and syrup and mix in to form a batter.
The batter should be wet but hard to mix at this point. Gradually add enough milk to achieve this consistency. Chop the 4 squares of 85% chocolate into roughly eights and add to the mix.
Bake in the preheated oven for 50 minutes or until golden brown and a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool in the tin before transferring to a cooling rack to cool fully before slicing and serving.