- Oil, for greasing
- 200g (7oz) self-raising flour
- 50g (1¾oz) cocoa powder, sifted
- 1 tsp bicarbonate of soda
- 100g (3½oz) butter, cubed
- 100ml (3½fl oz) semi-skimmed milk
- 3 large free-range eggs, beaten
- 250g (9oz) sweet potato, finely grated
- 1 large very ripe banana (approximately 125g or 4½oz peeled weight), mashed with a fork
- Preheat the oven to 200°C / 180°C Fan / 400°F or Gas mark 6.
- Grease and line the base and sides of a 23cm / 9in loose-based square cake tin with baking parchment.
- Mix the flour, cocoa and bicarbonate of soda in a large bowl. Make a well in the centre.
- Put the butter in a medium saucepan and melt over a low heat, stirring regularly. Take the pan off the heat and stir the milk into the melted butter. Add the eggs, sweet potato and banana, stirring vigorously.
- Pour slowly into the flour mixture, stirring constantly with a wooden spoon to form a thick batter. Pour into the prepared tin.
- Bake in the centre of the oven for 30-35 minutes, or until the cake is risen and just firm to the touch.
- Serve warm or leave to cool in the tin before cutting into squares.