Preheat oven to 180°C/160°C fan-forced. Grease a 6cm-deep, 19cm (base) square cake pan. Line base and sides with baking paper.

Place cake mix in a large bowl. Add eggs, butter and buttermilk. Using an electric mixer, beat on low speed for 30 seconds or until just combined. Increase speed to medium. Beat for 4 minutes or until mixture is pale and creamy. Stir in banana and walnuts. Pour into prepared pan.

Bake for 40 minutes or until a skewer inserted in the centre comes out clean. Cool in pan for 10 minutes. Turn onto a wire rack to cool completely. Spread top with frosting (see related recipe). Serve.