Start by making the curd. Put all the ingredients in a saucepan and whisk together. It will look like it has curdled but don’t worry – simply put the pan over a low heat and stir constantly until it is smooth and thick enough to coat the back of a spoon. Pass the mixture through a sieve into a bowl. Leave to cool, covered with cling film. Put in the fridge until ready to use, preferably overnight.
For the caramel sugar syrup, heat the caster sugar in a saucepan over a medium heat until it melts and starts to caramelise. Stir until smooth and a deep golden colour. Remove from the heat and carefully pour in 50ml boiling water. It will steam and spit a little, so take care. (If the sugar hardens, pop the pan back on the heat for 1-2 mins to melt again.) Stir, pour immediately into a heatproof jug and top up with a little extra boiling water (about 10ml) so you have 100ml of liquid in total. Leave on one side to cool.
To make the cake, heat oven to 180C/160C fan/gas 4. Grease 3 x 20cm sandwich tins, lining the bases and sides with baking parchment.
In a large bowl, beat together the butter and sugars with an electric whisk until light and fluffy. Mix in 1 tbsp flour, then add the eggs, a little at a time, beating after each addition until fully combined.
Mix together the remaining dry ingredients, the lemon zest and 1/2 tsp salt, then fold in to the butter mixture with the caramel sugar syrup. Divide the batter between the tins and level the tops with the back of a wet spoon. Bake for 25-30 mins or until a skewer inserted into the middle comes out clean. Allow the cakes to cool in the tins for 10 mins, then turn out onto a wire rack, peeling off the parchment. Leave to cool completely.
Make the caramelised lemon slices while the cakes are baking. Slice the lemon into 5mm thick slices, discarding the ends. In a shallow non-stick pan, bring 350ml water and the sugar to the boil. Add the lemon slices, boil for 10 mins, then reduce the heat to a simmer for 20-25 mins until the liquid has evaporated and the slices have caramelised. Remove from the pan and put on a non-stick A silicone mat or baking parchment to cool.
To assemble the cake. Place a sponge the right way up on a cake board or presentation plate. Top with an even layer of curd but don’t go right to the edge, leave about 1cm uncovered. Gently put the next sponge on top and repeat the process, putting the last sponge upside down on top.
To make the Italian meringue, put the sugar and 175ml water in a saucepan and B bring to a rolling boil over a high heat. Continue to boil until it reaches 115C on a sugar thermometer. Meanwhile, put the egg whites and cream of tartar in a large mixing bowl (a tabletop mixer is ideal if you have one). Whisk the egg whites to soft peaks (when lifting the whisk out, the peaks should slowly vanish back into the mixture). Whisk the egg whites at high speed and very slowly trickle in the hot syrup. The meringue will C begin to thicken and go glossy after about 10 mins. Continue to whisk until it is still just warm. Use the meringue immediately, as it is easier to work with while warm.
Using a palette knife, spread a thin layer of meringue around the side of the cake to level and square it up, then spread an even layer about 3mm thick on the top of the cake (see step-by-step for guidance). Put the remaining meringue in a large piping bag fitted with a small star nozzle and pipe vertical columns up the side of the cake, level with the top. Finally, pipe small meringue stars on the top of the columns. It’s easier to do this with the cake on a turntable.
Brown the meringue using a kitchen blowtorch. Finally, cut the caramelised lemon slices in half and use to decorate the top of the cake. Will keep for 3 days in the fridge.