1.Preheat oven to 180°C/350°F. Grease deep 20cm (8 inch) square cake pan; line base with baking paper.
2.Beat butter and sugar in medium bowl with electric mixer until light and fluffy. Beat in eggs, one at a time. Stir in banana, then combined sifted flours and soda, and nuts. Spread mixture into pan; bake about 1 hour.
3.Meanwhile, to make butterscotch syrup, combine sugar and butter in small saucepan; stir over heat until butter melts. Add the water and bring to the boil, stirring; remove from heat.
4.Stand cake in pan 5 minutes before turning, top-side up, onto wire rack set over tray. Drizzle hot syrup over hot cake.
Store cake, with or without syrup, in an airtight container, at room temperature, for up to 2 days. Cake without syrup can be frozen for up to 3 months. You will need 2 large overripe bananas for this recipe. Walnuts, pecans or macadamias work just as well as hazelnuts.