A dramatic-looking cake with three orange- and chocolate-flavoured layers, sandwiched with orange curd and frosted with orange buttercream. The caramel challenge is simplified here to a sugar-work waterfall, honeycomb rocks and a praline forest floor.
Heat the oven to 180°C/160°C fan/350°F/Gas 4.
Make the orange sponge mixture. Put the butter into a mixing bowl and beat with an electric whisk for 2–3 minutes, until very creamy. Add the sugar and mix for about 5 minutes.
Little by little, add the eggs, beating well after each addition. Add the orange zest and juice, sift the flour and baking powder on top, then carefully fold everything together until thoroughly combined. Mix in 2–3 drops of the orange food colouring to give a good but not vivid orange colour. Set aside.
Make the chocolate sponge. Put the cocoa into a heatproof bowl and stir in the boiling water to make a smooth paste. Leave to cool. Put the butter into another bowl, beat with the electric whisk until creamy, then incorporate the sugar followed by the eggs. Beat in the cocoa mixture, then fold in the sifted flour and baking powder until thoroughly combined.
Add alternate tablespoonfuls of the orange and the chocolate mixtures to the 3 tins until each tin is about half full. Don’t stir the two mixtures, just gently tap the tins on the worktop to settle the contents.
Bake for about 20–25 minutes, until each cake is well risen and springs back when lightly pressed. Loosen the cakes with a round-bladed knife, leave for 2 minutes to firm up in the tin, then carefully turn out onto a wire rack to cool.
Meanwhile, make the orange curd. Put the orange zest, orange juice, sugar and cornflour into a heatproof bowl and mix until smooth. Stir in the butter, then suspend the bowl over a pan of simmering water and stir continuously until the butter and sugar have melted. Remove the bowl from the pan (but leave the pan on the heat).
Strain the beaten eggs into the orange curd mixture, stirring continuously. Return the bowl to the pan and stir until the mixture is very hot and opaque and thick enough to coat the back of a wooden spoon. Remove from the pan, leave to cool, stirring frequently, then cover and chill for about 1 hour, until firm enough to spread.
Make the buttercream. Put the butter into a large mixing bowl and beat well with an electric whisk. Slowly whisk in the sifted icing sugar, beat in the zest, then gradually add the juice to make a smooth and light buttercream. Chill briefly, if necessary, so the mixture is easy to spread.
To assemble the cake. Set 1 sponge top-side down on a serving board and spread with a thick layer of orange curd. Put another sponge on top and spread with curd as before. Cover and chill the remaining curd (you can serve on toast another time). Top with the third sponge, top-side up, then cover the whole cake with a thin layer of buttercream to catch the crumbs and chill for 20 minutes or until very firm.
Spread the sides of the bottom sponge cake with another, thicker layer of buttercream. Add a couple of drops of food colouring to the remaining buttercream and mix thoroughly to a rosy-gold hue, then cover the sides of the middle sponge cake. Add a couple more drops of colouring to the buttercream to a dark orange, then cover the entire top sponge.
Carefully blend the joins between the buttercream layers to give a sunset effect. Chill the cake while you make the waterfall.
Put the sugar in an even layer in the base of a heavy, deep-sided frying pan. Set over a medium–low heat, and leave to melt, very gently, tilting the pan from side to side now and then.
As soon as the sugar starts to melt, very slowly draw it away from the sides of the pan with a wooden spoon, until you have a rich, dark golden liquid. Remove the pan from the heat, cover your hand, and slowly pour in the warm cream. (Take care: the mixture will splutter.) Carefully stir, then add the butter and return the pan to the heat. Stir until smooth, then transfer to a heatproof bowl and leave to cool.
Make the honeycomb rocks. Sift the ginger and bicarbonate of soda onto a piece of baking paper. Put the sugar and golden syrup into a deep, medium pan and set over a low heat. Warm gently for about 10 minutes, stirring occasionally, until the sugar has melted. Turn up the heat, then boil (without stirring) until it turns golden brown. Remove from the heat.
Protect your hand and quickly add the bicarbonate of soda and ginger mixture, whisking it in for a couple of seconds. The mixture will froth up and expand (take care). Quickly pour onto one side of the oiled baking sheet – don’t attempt to spread it out or touch it (or the baking sheet). Leave to cool and harden, then break into rock-like chunks.
To make the praline forest floor, put the pistachios in a small pan, cover with cold water and bring to the boil. Drain thoroughly then tip out onto the lined plate. Gently rub the pistachios to remove the papery skins, then set aside to dry.
Put the sugar into a small, heavy pan and gently melt, then boil without stirring, until the liquid turns a rich golden caramel colour. Remove from the heat and stir in the pistachios. Tip out onto the empty side of the oiled baking sheet. Leave until cold and hard, then break up and chop coarsely.
To assemble, transfer the caramel sauce to the small piping bag and snip the end. Starting towards the centre of the cake, pipe a river across the top and a waterfall down one side. Scatter the pistachio praline either side of the piped caramel, then decorate the top and base of the waterfall with the honeycomb rocks.