1.To make caramel sauce, stir ingredients in a small saucepan over low heat until sugar is dissolved and mixture is smooth.
2.Preheat oven to 180°C/350°F. Grease a deep 20cm (8-inch) round cake pan; line base and side with baking paper.
3.Beat butter and sugar in a small bowl with an electric mixer until light and fluffy. Beat in eggs, one at a time. Transfer mixture to a large bowl; stir in banana.
4.Combine soda and milk in small jug; stir into banana mixture. Stir in sifted flours, in two batches. Spread mixture into pan.
5.Bake cake about 50 minutes. Stand in pan 5 minutes; turn, top-side up, onto a wire rack to cool.
6.Serve slices of cake drizzled with warm sauce.
The cake will keep in an airtight container, at room temperature, for up to 4 days, and can be frozen for up to 3 months. Store any unused sauce in the refrigerator.