Preheat the oven to 350°F (175°C) and grease a standard loaf pan.
In a large bowl, mash the bananas with a fork until mostly smooth. Add the softened butter and brown sugar, and mix until well combined.
Add the egg and vanilla extract, and stir until evenly incorporated.
Add the flour, baking powder, salt, cinnamon, nutmeg, cloves, and ginger. Mix until just combined - don't overmix. Fold in most of the chopped walnuts, saving a handful for the top.
Pour the batter into the prepared loaf pan and smooth the top. Sprinkle the remaining walnuts over the surface.
Bake for 40-45 minutes, until a toothpick inserted into the center comes out clean or with just a few moist crumbs.
Let the loaf cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely before slicing.