Three layers of light sponge, flecked with poppy seeds, sandwiched together with a caramel, toasted walnut and prune filling – then covered with a light-as-air caramel buttercream. Golden caramel shards and more walnuts add the finishing touch.
Heat the oven to 190°C/170°C fan/375°F/Gas 5.
Make the sponges. Whisk the egg whites to soft peaks in a stand mixer fitted with the whisk attachment (or use a hand-held electric whisk). Quickly whisk in half the sugar to make a stiff, glossy meringue. Set aside.
Put the egg yolks in a clean bowl and add the remaining 100g of sugar, along with the vanilla extract, and whisk with an electric whisk until very thick and mousse-like and the whisk leaves a ribbon-like trail when lifted. Then whisk in the poppy seeds.
Carefully fold in one third of the sifted flour, then one third of the meringue. Repeat twice more until you’ve used up all the flour and meringue and no streaks of egg white or specks of flour remain.
Slowly pour the cooled melted butter around the sides of the bowl, then fold in. Divide the mixture equally between the 3 prepared tins.
Bake the sponges for about 17–20 minutes, until golden, well risen and springy when gently pressed. Loosen the sponges, then turn them out onto a wire rack and leave to cool.
Make the filling. Tip the walnut pieces into a baking dish and toast them in the oven for about 10 minutes, until golden. Leave to cool. Put the stoned prunes into the bowl of a food processor and blitz to a thick purée. Add the walnut and blitz briefly to chop until medium-fine. Transfer to a bowl and set aside.
To make the caramel sauce, put the butter, golden syrup and caster sugar into a medium pan and stir over a low heat until melted and smooth. Meanwhile, in another pan or in the microwave, heat the cream until it’s almost boiling, then leave to cool.
Bring the golden syrup mixture to the boil and cook, stirring, for about 4 minutes, or until it turns a rich caramel colour. Remove from the heat, protect your hand and very carefully add the hot cream. (Take care: the mixture will splutter.)
When the foaming subsides, return the pan to a low heat and whisk continuously with a wire whisk for 2 minutes, until smooth and thick. Pour into a heatproof bowl. Leave to cool, then cover and chill for 20 minutes, or until set and slightly colder than room temperature.
To make the caramel buttercream, beat the butter using an electric whisk for about 5 minutes, until very light. Gradually whisk in the caramel sauce until very smooth and light. Separate out 100g of the buttercream and mix this into the prune and walnut filling. Add the sifted icing sugar to the remaining buttercream and whisk for about 3 minutes, until very light and fluffy. Transfer one third to a separate bowl.
To assemble the cake, set one sponge top-side down on a serving plate and spread with half the walnut and prune filling. Place another sponge on top and spread with the remaining filling. Cover with the third sponge, top-side up.
Evenly spread the top and sides of the cake with the reserved one-third of the buttercream (to catch the crumbs). Chill for 10 minutes, or until firm.
Spread the remaining two thirds of buttercream over the entire cake and chill until firm.
Make the caramel for the shards. Put the sugar into a medium pan (not non-stick) and shake the pan so it settles in an even layer. Spoon 4 tablespoons of water over the sugar. Heat very gently over a low heat, stirring occasionally, and brushing down the sides with a heatproof pastry brush, until the sugar has dissolved.
Increase the heat and boil without stirring until the syrup turns a rich, golden-brown caramel colour. Remove from the heat and very carefully pour about three-quarters of it on the oiled baking sheet in a thin, even layer (it won’t fill the entire sheet).
Add the walnut halves to the remaining quarter of caramel in the pan, and stir to coat. Using a fork, carefully remove the nuts from the pan and set them on the baking sheet, to one side of the caramel. Leave to set.
Break the set caramel into shards. Press the shards into the buttercream around the sides of the cake and decorate the top with the caramelised walnuts.