Traybakes are a fantastic way to feed large groups of cake-hungry visitors. The citrus in this crowd-pleaser is balanced with the gentle spice of cardamom.
Preheat the oven to 200°C/180°C fan/400°F/Gas 6.
First, make the joconde sponge. Place the egg whites in the bowl of a stand mixer fitted with the whisk attachment. Add 90g of the caster sugar and whisk at high speed for 10–15 minutes, until the mixture forms stiff peaks. Spoon into another bowl and set aside.
Wipe out the mixing bowl and add the ground almonds, the rest of the sugar, and the whole eggs and whisk for about 5–7 minutes, until the mixture is thick and forms ribbons when you lift the whisk. Using a large metal spoon, gently fold the meringue mixture into the almond mixture until combined.
Carefully fold in the flour, lemon zest, ground cardamom (or cardamom extract) and cooled melted butter, until evenly combined, but taking care not to knock out the air. Spread the mixture into the prepared baking tins, levelling it out with a palette knife, and bake for 10–11 minutes, until pale golden and just firm to the touch.
To make the lemon and cardamom drizzle, put the lemon juice, sugar, cardamom, and lemon extract in a pan with 25ml of water and bring to the boil over a high heat. Reduce the heat to low and simmer for 3–4 minutes, until slightly syrupy. Remove from the heat and leave to cool.
Meanwhile make the lemon curd. Place all the ingredients except the butter in a medium pan over a medium heat, stirring constantly, for 5–7 minutes, until it starts to thicken and coats the back of a spoon. Remove from the heat and stir in the butter, then pour the mixture into a shallow bowl. Cover the surface with cling film, allow to cool to room temperature, then place in the fridge to cool completely.
To make the Italian buttercream, mix the caster sugar with 60ml of water in a medium pan and place over a medium heat. Dissolve the sugar without stirring (brush down the sides with a heatproof pastry brush from time to time), then check the temperature of the syrup. When it reaches about 110°C, place the egg whites in a stand mixer fitted with the whisk attachment and whisk on high speed until the whites form soft peaks. Keep the whisk running.
When the sugar syrup reaches 121°C, slowly and very carefully (it will be hot), pour it in a thin, steady stream into the egg whites. Continue whisking until the outside of the bowl is at room temperature and the meringue is very thick and glossy.
Whisk in cubes of both butters, a few cubes at a time, until the mixture is smooth, thick and glossy. Add the lemon extract and whisk to combine. Cover and chill until firm enough to spread.
To assemble, brush 1 sponge with the lemon syrup. Take one third of the buttercream and mix in 2–3 tablespoons of lemon curd. Spread this over the sponge. Place the second sponge on the top and drizzle with the syrup.
Add cardamom extract to three quarters of the remaining buttercream and spread it smoothly on the top of the second sponge. Chill in the fridge or freezer for 15 minutes.
Trim the ends and sides off the cake, then slice it into 16 equal rectangles. Place a little drop of lemon curd in the centre of each slice. Fill the piping bag fitted with a star nozzle with the rest of the buttercream and pipe 2 rosettes either side of the drop of curd. Top each drop of lemon curd with a green cardamom pod, if you wish.