1. Preheat oven to 180C/350F/gas 4.
2. Place the orange pieces in a saucepan and cover with water. Bring to the boil and simmer for 30 minutes until the orange is softened.
3. Drain, transfer to a food processor and whizz until smooth. Set to one side.
4. Line a square tin with baking parchment. In a bowl mix together the orange pulp, sugar, egg whites and yogurt.
5. Sift the flour, baking powder and cocoa and lightly fold together. Transfer the mixture to the prepared tin and gently level off.
6. Bake for approximately 30 minutes until risen and firm. Allow to cool in the tin for 5 minutes then transfer to a wire rack to cool.
7. Cut into 9 equal squares and lightly dust with icing sugar.
- 1 orange, peel washed and then cut into 8 pieces
- 115g/4oz muscavado sugar
- 2 egg whites, lightly whisked
- 30ml/2tbsp natural yogurt
- 115g/4oz plain flour
- 25g/1oz cocoa powder
- 10ml/2tsp baking powder
- icing sugar to dust