A classic sponge flavoured with orange and lemon zest and baked in a ring mould, then imaginatively decorated with small caramel-dipped choux buns filled with an apple-purée crème diplomat.
Heat the oven to 220°C/200°C fan/425°F/Gas 7. Prepare the cake mould: brush the inside of the mould all over thoroughly with melted butter. Sprinkle the sugar onto the melted butter to coat, then shake out the excess.
Make the sponge. Sift the cornflour and flour into a small bowl and set aside. Put the egg whites in the bowl of a stand mixer fitted with the whisk attachment. Add the salt, then whisk until the egg white stands in stiff peaks. Carefully transfer to a clean bowl and set side.
Whisk the egg yolks in the mixer, then add the icing sugar and whisk again until the mixture is thick and the whisk leaves a ribbon-like trail when lifted. Fold in both zests and the yuzu (if using).
Gently fold in one third of the sifted flours, followed by one third of the whisked egg whites. Repeat twice more until thoroughly combined. Pour the mixture evenly into the prepared mould.
Bake the sponge for 5 minutes, then reduce the temperature to 160°C/140°C fan/315°F/Gas 2–3 and bake for a further 30–35 minutes, until well risen, golden and a skewer inserted mid-way between the side and centre comes out clean. Leave to firm up for 5 minutes, then carefully unmould onto a wire rack to cool completely.
Make the choux buns. Increase the oven temperature to 200°C/180°C fan/400°F/Gas 6. Put the butter, salt and sugar into a medium pan with 125ml of water, over a low heat, and heat gently to melt the butter.
Increase the heat and quickly bring the mixture to a boil, then tip in all the flour. Take the pan off the heat and beat furiously with a wooden spoon to a smooth, heavy dough.
Put the pan back on a low heat and beat gently for 1 minute, until the dough forms a smooth, glossy ball. Tip the dough into a large heatproof bowl and cool until barely warm.
Using an electric whisk, beat in the eggs a little at a time, beating well after each addition, to a shiny, smooth, pipeable paste. Transfer to the large disposable piping bag and snip the end to make a 2.5cm hole.
Pipe mounds about 3cm across and 2.5cm high on the lined baking sheet, spacing them well apart. Aim for at least 36 buns, and ideally a few extra to allow for mistakes. Gently flatten any peaks with a wet finger.
Bake the buns for 25 minutes, then reduce the oven to 180°C/160°C fan/350°F/Gas 4. Open then quickly close the oven door to release the steam, then bake for a further 5 minutes, until the pastry is crisp and golden.
Turn out the buns (so that they are upside down) onto a wire rack and immediately make a small hole in the flat base of each bun. Leave like this to cool.
Make the crème diplomat. Heat the apple purée in a medium pan over a low heat with half the sugar and all the vanilla paste, until just hot.
In a heatproof bowl, combine the remaining sugar with the egg yolks and cornflour to make a smooth, thick paste. Stir in the hot purée, then return the mixture to the pan.
Stir over a medium–low heat until the mixture comes to the boil and is smooth and thick. Transfer to a clean heatproof bowl, leave to cool, then cover and chill thoroughly.
Whip the chilled cream to soft peaks, then fold into the chilled apple mixture. Spoon into the large piping bag fitted with the small nozzle, and pipe into each choux bun through the hole in the base. Put the cooled sponge on a serving plate.
Make the caramel. Put the sugar and 4 tablespoons of water into a small, heavy-based pan over a low heat. Allow the sugar to melt, occasionally brushing down the sides, then bring it to the boil and cook (without stirring) until it starts to turn amber.
Remove the pan from the heat and, working very quickly and carefully, lightly dip in the rounded top and sides of each bun using kitchen tongs (not your fingers!).
Then (still using the tongs) position the flat base of each bun on the sides of the cake – starting from the base and building up two rings of buns that stick together at the sides. Dip every third or fourth bun in caramel and then into either the pearl sugar or the freeze-dried strawberries before attaching to the cake. Place the caramel back on the heat from time to time to keep it fluid.
Use any remaining caramel to make angel hair for the top of the sponge: dip the tines of a fork in the caramel, lift the fork high up out of the pan to create long, fine strands of caramel, then flick the fork over the top of the cake. Serve as soon as possible.