Preheat oven to 160°C fan / 180°C / 350°F
Grease and line a 9×5-inch loaf tin, leaving an overhang for easy removal
In a medium bowl, sift flour, baking powder, bicarbonate of soda and salt. Set aside.
Whisk sugar and eggs together in a large bowl. Mix in mashed bananas, yoghurt, vanilla extract and melted butter.
Gently fold dry ingredients into wet ingredients.
Mix until just combined (don’t overmix).
Remove about 250ml of batter into a medium mixing bowl.
Mix in cocoa powder and coffee.
Alternate spoonfuls of banana and chocolate batter into the tin.
Use a knife or skewer to gently swirl (don’t overdo it).
Sprinkle chilled crumb topping evenly over the top.
Bake for 60 minutes, or until a skewer inserted in the centre comes out clean.