Preheat oven to 350°F (175°C) and grease a loaf pan.
In a large bowl, combine mashed bananas, softened butter, sugar, coffee, eggs, and vanilla extract.
In a separate bowl, whisk together flour, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture and stir until combined.
Pour the batter into the prepared loaf pan.
For the streusel, combine walnuts, brown sugar, flour, butter, and cinnamon in a bowl.
Sprinkle the walnut streusel evenly over the batter.
Bake for 55 minutes, or until a toothpick inserted into the center comes out clean.
Allow to cool in the pan for 10 minutes before transferring to a wire rack.