Preheat the oven to 180°C (350°F). Grease a 6" x 3" loaf tin and line it with butter paper.
Mash the banana well with a fork in a large bowl.
Add the brown sugar and oil, and mix well.
Sift in the buckwheat flour, baking soda and cinnamon.
Add the curd and vanilla, and mix until just combined.
Fold in the walnuts, cashews and raisins.
Pour the batter into the tin and level the top with a spatula or the back of a spoon.
Sprinkle a mix of chopped walnuts and brown sugar over the top.
Bake for 25–30 minutes, until a toothpick comes out clean.
Cool for 10 minutes, loosen the sides with a knife, turn the cake out and peel off the paper.
Slice and serve.