1 large very ripe banana (approximately125g/4陆oz peeled weight), mashed with a fork
250g/9oz sweet potato, finely grated
1 tsp bicarbonate of soda
50驳/1戮辞锄 cocoa powder, sifted
oil, for greasing
200g/7oz self-raising flour
100驳/3陆辞锄 butter, cubed
3 large free-range eggs, beaten
100ml/3陆fl oz semi-skimmed milk