Preheat the oven to 170C/325F/Gas 3. Grease two 23cm/9in round cake tins and line the sides and bottom with parchment paper.
For the torte, in a mixing bowl beat the egg yolks and caster sugar until light and fluffy. Add the crème fraîche, lemon zest and the fiori di Sicilia or arome di panettone. Mix together and sift in the flour and baking powder (it is essential to sift chestnut flour as it clumps together easily). In a clean bowl, whisk the egg whites to stiff peaks. In three batches fold the eggs whites into the batter using a metal spoon. Pour the batter into the prepared tins, smooth the tops and bake in the oven for 23–30 minutes.
For the topping, simmer 250ml/9fl oz water with the caster sugar in a pan for two minutes until a syrup forms. Remove the rind from the lemons, orange and limes and cut into fine strips (julienne). Segment the oranges. Add the rind strips to the syrup, simmer for three minutes, remove from the heat and set aside to cool.
Remove the torte from the oven and cool in the tins for five minutes before turning out on to a cooling rack to cool completely.
Lightly paint each verbena leaf with a little egg white, dip into caster sugar and place on a lined tray. Dry the verbena leaves in the oven at the lowest possible oven temperature for approximately 20 minutes, until crisp. Set aside to harden.
For the lemon cheese icing, whisk the softened butter for two minutes. Add the icing sugar and lemon oil extract. Mix well before adding the cream cheese, then beat for 5-6 minutes until light and creamy. Cover and transfer to the fridge until needed.
To assemble the torte, spread the chestnut spread on both torte layers. Spread a layer of lemon icing on top of the chestnut spread on both cakes, leaving a 1cm/½in gap at the edge.
On the bottom layer of the torte arrange the clementine segments around the edge of the cake with their ‘backs’ facing outwards and 1cm/½in space between each one.
Fill a piping bag with the remaining lemon icing and pipe a rosette into the gaps between the clementines. Arrange two concentric rings of orange segments and pipe some icing in between the rings (to avoid holes).
Place the second layer of torte on top, with the lemon icing facing upwards. Fill the 1cm/½in border with piped rosettes of lemon icing.
Drain the citrus rinds and pat dry with kitchen towel. On each lemon icing rosette apply a small amount of the citrus rinds. Arrange the remaining orange segments in centre of cake. Pipe small rosettes of cream in any gaps, and scatter some of the mixed zest over. Place crystallised verbena leaves on top in a radiating pattern.