Rich, coffee mousse, crunchy biscuit and lots of hazelnuts make these biscuit bars an irresistible combination. The subtle cardamom spice provides a delicious flavour twist.
Begin with the shells. Melt 350g of the chocolate in a heatproof bowl set over a pan of gently simmering water until it reaches 55°C on the sugar thermometer. Remove one third of the melted chocolate to a separate bowl, and put the remaining 100g solid chocolate into the remainder over the pan of water. Stir it in and cool the chocolate to 28°C, then add back in the small amount of melted chocolate and mix until the chocolate reaches 30°C. Remove from the heat and, using the pastry brush, use the tempered chocolate to coat the insides of the 12 buche moulds, then chill to set.
Make the shortbread. Beat the butter and both sugars together in a stand mixer fitted with the beater, on medium speed for about 5 minutes, until pale and creamy. Add both flours and beat to combine.
Blitz the hazelnuts in a food processor until coarsely ground. Add them to the mixture in the stand mixer, along with the egg yolk and vanilla, and mix to form a soft dough. If necessary, wrap the dough in cling film and chill for about 15 minutes, until firm enough to roll. Measure the base of a buche-mould hollow, then use the card to cut a template to the size for each biscuit base.
Roll out the dough on a lightly floured work surface to about to 6mm thick. Cut it into fingers using the template. Place the shortbreads on a lined baking tray, freeze for 10 minutes. While the shortbreads are freezing, heat the oven to 160°C/140°C fan/Gas 3.
When the shortbreads are ready, bake them for 12–15 minutes, until just turning golden. Leave to cool on the tray for 10 minutes, then transfer to a wire rack to cool completely.
Make the coffee and cardamom mousse. Soften the gelatine leaves in 3 tablespoons of water. Place the egg yolks and sugar together in a small bowl and beat with a balloon whisk for 1–2 minutes, until creamy. In a separate small bowl, whip the cream until thick.
In a clean, grease-free bowl, using an electric hand whisk, whisk the egg whites to stiff peaks. Dissolve the coffee in the boiling water, then squeeze out the gelatine and stir it into the hot coffee with the liqueur and cardamom until dissolved. Whisk the coffee mixture into the egg yolk mixture, and alternately fold in the egg whites and the whipped cream, until no streaks remain.
Pour the mousse into the chocolate shells until they are half full, then freeze for 20 minutes to set. (You will have a little mousse left over – pour it into a dessert glass and chill to set, then have it for dessert topped with leftover caramelised hazelnuts and ganache.)
Make the caramelised hazelnuts. Dissolve the sugar in a small pan over a medium heat, shaking the pan gently without stirring. Increase the heat to high and allow the sugar to caramelise around the edge, shaking the pan occasionally until all the sugar turns a good golden brown. As soon as the sugar is ready, remove it from the heat (the caramel will turn bitter if you leave it too long).
Add 12 whole hazelnuts, stir to coat, then remove them with a fork to a lined baking tray. Add the chopped hazelnuts to the remaining caramel. Pour onto a separate baking tray and allow to set, then bash with the end of a rolling pin to break into fine pieces. Spoon a little of the caramelised hazelnut crumbs into each mould. (You will have some left over.)
Make the filling. Melt the milk chocolate in a heatproof bowl set over a pan of gently simmering water. Stir in the chopped hazelnuts, remove the bowl from the heat and allow the mixture to cool for 10 minutes. Spoon a little filling into each mould.
Place the shortbread rectangles on the top of the filling in each mould, then place in the fridge for 5 minutes to firm up. Finally, spread a thin layer of the tempered chocolate (re-melt it if necessary) over the top to encase the filling, using a palette knife to remove any excess. Leave the bars to set in the fridge for at least 2 hours.
Make the ganache. Pour the double cream into a small pan over a low heat. Add the butter and ground cardamom and bring the mixture to a simmer. Add the chocolate and stir until melted, then pour into a bowl and place in the fridge for at least 1 hour, or until firm enough to pipe.
Once the bars have been chilling for 2 hours, turn them out of the moulds onto a plate. Place the ganache in a piping bag fitted with a small star nozzle and pipe tiny rosettes along the length of each bar (you will have some ganache left over). Top each bar with a coffee been, and a whole hazelnut flaked with gold leaf.