- Wash and peel the potatoes and cut into pieces. Cook them in salted, boiling water then drain them and crush with a fork. Refrigerate overnight.
- Sieve the flour, icing sugar and salt into the bowl with the mashed potatoes. Mix together.
- Cut the butter into cubes and mix into the mashed potato mixture.
- Preheat the oven to 180°C/Gas 4.
- Spread the mixture over a baking tray lined with baking paper and refrigerate for 30 minutes.
- Place the tray in the oven and cook for 20 minutes. Meanwhile, prepare the lemon mixture/batter.
- Beat the eggs with 200 g of sugar until frothy.
- Grate the lemon zest and dilute the potato starch with the juice from the 3 lemons. Mix the lemons with the eggs and heat the mixture over a medium heat for 5 minutes, stirring well, until thickened.
- Pour the mixture over the sponge and return to the oven for 10-15 minutes.
- Refrigerate and sprinkle with icing sugar and cut into 3-4 cm pieces just before serving.
For an exotic fruit version, replace the lemon with passion fruit.