Making a chocolate collar is simpler than it looks, so don’t be afraid to give it a go. If you’re short on time, though, the cake is just as delicious without it.
Preheat the oven to 180°C/160°C fan/350°F/Gas 4. Put the butter and sugar in the bowl of a stand mixer fitted with the beater attachment and beat until pale and creamy. Add the eggs, a little at a time, adding a little flour if the mixture curdles. Mix in the vanilla paste, orange extract, orange zest and Cointreau. Then, gently fold in the flour, until combined.
Divide the mixture equally between the 3 cake tins. Bake for 25 minutes, until golden brown and just firm to the touch. Remove from the oven (but leave the oven on), then turn out to cool on a wire rack.
Meanwhile, make the sugar syrup. Place 50ml of water, the sugar and the Cointreau in a small pan over a high heat and bring to the boil. Reduce the heat to low and simmer for 3–4 minutes, until slightly syrupy. Take off the heat and leave to cool, then using a pastry brush, brush the sponges with the cooled sugar syrup.
Make the chocolate cream filling. Spread the hazelnuts on a baking sheet and roast for 5 minutes until pale golden brown. Place in a food processor and blitz for a few minutes until they turn into a butter.
Melt the milk chocolate in the microwave or in a small bowl over a pan of gently simmering water, until just melted. Add to the food processor with the sugar, cocoa, coconut oil, vanilla, and salt. Blend for another 1 minute, until all the ingredients are combined.
Whisk together the double cream and mascarpone in large mixing bowl with an electric hand whisk, until firm. Spoon 2 tablespoons of the chocolate–hazelnut mixture into the whipped cream. Whisk to combine, taste, then add more chocolate–hazelnut if you like. Put the cream mixture in a medium disposable piping bag and snip the end to make a 1cm hole. Set aside the remaining chocolate–hazelnut mixture.
Make the orange cream filling. Place all the orange cream ingredients into a large bowl and whisk with an electric hand whisk until firm. Put half the mixture in the second medium disposable piping bag and snip the end to make a 1cm hole. Reserve the rest.
To assemble the cake, spread a little orange cream onto a cake board or cake plate and then place 1 sponge on top. Using a palette knife, spread about 1 tablespoon of the remaining chocolate–hazelnut mixture on top. Pipe the chocolate cream on top, starting at the middle and spiralling outwards to 1cm from the edge. Pipe the orange cream around the edge. Stack the second sponge on top and repeat, using the remaining chocolate cream, and the rest of the orange cream in the piping bag. Place the third sponge on top, spread it thinly with the remaining chocolate–hazelnut mixture, then use a palette knife to cover the top and sides of the cake completely with the leftover half of the orange cream. Place in the fridge.
Make the chocolate collar. Melt the white chocolate in the microwave in 15 second bursts (or in a bowl over gently simmering water). Divide the melted chocolate equally into 2 bowls and add black food colouring to 1 bowl to turn the chocolate grey.
Place the 2 strips of acetate on the worktop. Using a small spoon, thickly splatter melted white chocolate onto both acetates, in the style of Jackson Pollock. Place in the fridge for 10 minutes, to set, then remove and spread the grey chocolate over the top to about 2mm thick. Refrigerate for about 5 minutes, until firm but pliable. Wrap the collars around the cake, pressing firmly against the sides of the cake. Return the cake to the fridge for 15 minutes, or until ready to serve.
Make the hazelnut brittle. Melt the caster sugar in a large heavy-bottomed frying pan over a medium heat, shaking the pan occasionally (but not stirring), until the caramel is a deep golden colour. Add the chopped hazelnuts and swirl the pan to coat the nuts. Pour out onto the small, lined baking sheet. Leave to cool.
Place the 25g of drizzling white chocolate into a bowl and melt in the microwave in 10 second bursts. Drizzle the white chocolate over the brittle. Once set, break the brittle into shards and arrange them on top of the cake. Carefully peel away the acetates and serve.