* 2 small thin-skinned lemons, or 1 large one
* 275g (10oz) softened butter, plus extra for dusting
* 275g (10oz) caster sugar
* 275g (10oz) self-raising flour
* 2 level tsp baking powder
* 4 eggs
For the filling and icing
* 50g (2oz) softened butter
* 175g (6oz) icing sugar
* 250g (9oz) full-fat mascarpone
* 175g (6oz) fondant icing sugar
* 1 thin-skinned lemon
* 50g (2oz) caster sugar
1. Grease two 20cm (8in) round sandwich tins and line the bases with baking paper.
2. First make the lemon decoration. Peel long strips of lemon rind then cut into fine strips (keep the lemon to juice it for the icing). Place the peel in a saucepan with half the caster sugar, cover with boiling water and boil for one minute. Drain and pat dry, then place on baking paper.
3. Scatter over the remaining sugar and dry out in a low oven set at 110°C/90°C fan for about one hour, until crispy. Cut the lemon in half and squeeze the juice, then set aside for the icing. Increase the oven to 180°C/160°C fan.
4. To make the cake, place the two whole lemons in a small saucepan, cover with water, bring to the boil and simmer for 20 minutes or until very soft and tender. Drain and cut the lemons in half and remove any pips.
5. Place the boiled lemons in a food processor and process to a smoothish pulp but still with chunky bits. Transfer to a small bowl. Add all the remaining cake ingredients to the food processor and blend until smooth. Lift out the blade and stir in just over half the processed lemon pulp.
6. Divide the mixture evenly between the two prepared tins and bake for 30 minutes or until golden brown and just shrinking from the edges of the tins.
7. Leave to cool for five minutes then turn the cakes out, peel off the paper and leave to finish cooling on a wire rack.
8. To make the lemon filling, place the butter and icing sugar in a bowl and mix till smooth and creamy (you could also do this in a food processor). Add the mascarpone, mix again, then add the reserved lemon pulp and mix until just combined.
9. Cut both cakes in half horizontally so you have four layers. Spread the lemon cream evenly between three of the layers, stacking them on top of each other. Place the remaining layer on top.
10. Mix the fondant icing sugar with one or two tablespoons of the reserved lemon juice to make a thick pouring consistency. Pour the icing over the top of the cake, allowing it to drizzle down. Sprinkle the candied lemon rind on top.
The baked cake will keep for up to four days in a tin.
Freeze the unfilled baked sponges for up to three months.
More from Mary Berry Cooks: Mary Berry Cooks by Mary Berry (BBC Books, hardback £20) Photography by Georgia Glynn Smith.