For the decoration, using a zester, peel long strips of lemon rind. If you haven’t got a zester, thinly peel the lemon, then cut into fine strips. Set aside the remaining lemon.
Place the peel in a saucepan, cover with boiling water and half the caster sugar and boil for one minute. Drain and pat dry, then put onto non-stick parchment paper. Scatter over the remaining sugar and toss to coat the lemon peel. Arrange on the non-stick baking parchment and leave in a warm place to dry out for as long as possible or overnight, until crisp. If you are in a hurry, dry out in a low oven 110C/225F/Gas ¼ for 30 minutes.
Cut the zested lemon in half and squeeze the juice. Set it aside to make the icing.
For the cake, place the two whole (or one large) lemons in a small saucepan, cover with water, bring to the boil and simmer for 20 minutes, or until very soft and tender. Drain, cut the lemons in half and remove any pips.
Preheat the oven to 180C/350F/Gas 4 (160C fan).
Place the lemons, including the skin, in a food processor and process until smoothish, but still with some chunky bits. Put the mixture in a small bowl.
Add all the remaining cake ingredients into the food processor and blend until smooth. Lift out the blade and stir in just over half of the lemon pulp. Set the rest aside.
Divide the mixture evenly between the two tins and bake for 30 minutes, or until golden-brown and just shrinking from the edge of the tins.
Leave to cool for five minutes, then turn the cakes out, peel off the baking parchment and leave to finish cooling on a wire rack.
For the lemon filling, blend the butter and icing sugar in a food processor until smooth and creamy. Add the mascarpone, blend again and then add the lemon pulp and blend until just mixed.
Cut both cakes in half so that you have four layers. Spread the lemon cream evenly between the layers, stacking them on top of each other.
Mix the fondant icing sugar with 1-2 tbsp lemon juice (from the lemon used to make the candied zest decoration) 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.