Preheat the oven to 160°C. In a bowl, mix the flour with the cocoa powder.
Prepare the loaf tin by dusting it with a little of the flour and cocoa mixture.
Add the baking soda and sugar to the bowl with the flour and cocoa powder and mix.
In a large bowl, add the bananas and mash them with a fork. They should be truly ripe; if their peel is black, even better.
Add the coffee, sunflower oil and yogurt, and mix.
Gradually add the flour and cocoa mixture, add the walnuts and mix.
Transfer the batter to the prepared loaf tin and bake at 160°C for at least 45 minutes, until the cake is fully baked.
Let it cool and remove it from the tin.
Serve with melted dark chocolate on top.