Peel long strips of lemon rind using a zester or peeler.
Boil peel in water with 25g sugar for one minute, drain, and coat in remaining sugar.
Dry the peel overnight or in a 110°C oven for 30 minutes until crisp.
Squeeze juice from the zested lemon and set aside.
Simmer whole lemons in water for 20 minutes until very soft.
Preheat the oven to 180°C.
Process boiled lemons into a pulp in a food processor.
Blend remaining cake ingredients until smooth and fold in half the lemon pulp.
Divide batter into two tins and bake for 30 minutes at 180°C.
Cool cakes for five minutes before transferring to a wire rack.
Blend butter, icing sugar, mascarpone, and remaining lemon pulp for the filling.
Halve each cake horizontally to create four total layers.
Spread lemon cream between layers and stack them.
Whisk fondant icing sugar with reserved lemon juice and pour over the cake.
Garnish the top with candied lemon peel.