• 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


  1. Preheat the oven to 200°C / 180°C Fan / 400°F or Gas mark 6.
  2. Grease and line the base and sides of a 23cm / 9in loose-based square cake tin with baking parchment.
  3. Mix the flour, cocoa and bicarbonate of soda in a large bowl. Make a well in the centre.
  4. 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.
  5. Pour slowly into the flour mixture, stirring constantly with a wooden spoon to form a thick batter. Pour into the prepared tin.
  6. Bake in the centre of the oven for 30-35 minutes, or until the cake is risen and just firm to the touch.
  7. Serve warm or leave to cool in the tin before cutting into squares.