This light, lemon sponge cake is decorated with pink-tinted macaroons, sandwiched together with a rich citrus buttercream.
Heat the oven to 180°C/160°C fan/350°F/Gas 4. Brush the sides and base of the greased and lined tin with more melted butter, then sprinkle over the sugar, swirling to coat. Shake out the excess sugar.
Make the sponge. Sift the flours into a small bowl and set aside. Put the egg whites into the bowl of a stand mixer fitted with the whisk attachment. Add the salt and whisk to soft peaks. Whisk in 115g of the caster sugar to make a stiff, glossy meringue. Carefully transfer to a clean bowl and set aside.
Put the egg yolks in the mixer bowl and add the remaining sugar. Whisk until very thick and the whisk leaves a ribbon-like trail when lifted.
Finely grate the zest of both lemons into the bowl and briefly whisk in. Slowly add the juice from 1 lemon while whisking on a medium speed – the mixture will collapse a little and become less thick.
Gently fold the meringue into the yolk mixture in 3 batches. Sift the flour mixture again into the bowl and very carefully fold in until combined. Transfer the mixture to the prepared tin and level the surface. Sprinkle with caster sugar.
Bake for 25–30 minutes, until well risen, golden brown and a skewer inserted into the centre comes out clean. Leave to firm up for 1 minute, then loosen the sponge, if necessary, and gently turn out onto a wire rack to cool completely. Turn off the oven.
Make the macaroons. Blitz the ground almonds and icing sugar in a food processor, then sift into a large mixing bowl (discard any bits left in the sieve).
Whisk the egg whites with a small amount of pink food colouring on a slow speed to combine, then on a high speed until they stand in soft peaks. Whisk in the caster sugar and continue whisking until the meringue stands in stiff peaks.
Spoon the meringue into the almond mixture and carefully stir together until just combined and shiny.
Transfer the mixture to a large piping bag fitted with the plain nozzle, then pipe round discs of about 3.25cm on the lined baking sheet, spacing them well apart. You’ll need 40 macaroons (20 pairs), but make a few spares.
Gently flatten any peaks in the centre of the macaroons with a wet finger and leave for about 1 hour to allow a matt skin to form. Heat the oven to 150°C/130°C fan/300°F/Gas 2.
Bake the macaroons for 20 minutes, rotating after 10 minutes for an even bake. Don’t allow them to start to turn golden. Leave to cool, then peel the macaroons away from the baking paper.
Make the St Clements curd. Put the sugar and butter into a large heatproof bowl. Add the citrus zests, plus 90ml of squeezed lemon juice and 90ml of squeezed orange juice.
Suspend the bowl over a pan of boiling water and stir until the butter and sugar have completely melted. Remove the bowl from the heat.
Strain the beaten egg through a sieve into the melted mixture. Stir well, then return the bowl to the pan and stir continuously, until the curd thickens to coat the back of the spoon. Transfer to a clean bowl, cool, then cover and chill.
To make the buttercream, beat the butter and icing sugar with an electric hand whisk until very light and creamy. Gradually beat in enough of the chilled curd (about 5 or 6 tablespoons) to give a good flavour and piping consistency.
To assemble the cake, transfer the buttercream to the large piping bag fitted with the star nozzle. Pipe a neat swirl of buttercream over the flat base of 20 macaroons, then sandwich with the other 20. Set the cake on a serving platter and surround with the macaroons. Serve as soon as possible.