Bitter Chocolate and Chestnut Truffle Cake with Candied Chestnuts

First, prepare the cake base. Preheat the oven to 180°C/Gas Mark 4 and grease, then line a 28 x 18 x 4cm baking tin with non-stick baking paper. Set aside. Place the rum and chestnuts in a small saucepan and bring to the boil over a medium heat, then simmer for about 1 minute – just long enough to warm the nuts and help them to absorb the rum flavour. Remove from the heat and set aside for 10 minutes. Put the egg whites into a bowl and whisk until foaming, then add the sugar and whisk together until the mixture forms soft peaks. Set aside.

Half-fill a saucepan with water and bring up to a gentle simmer. Choose a heatproof mixing bowl that will sit comfortably over the saucepan without slipping down inside the pan, and place the bowl over the pan (you also want to make sure that the bottom of the bowl does not come into contact with the simmering water underneath). Put the chocolate and butter into the bowl and stir gently until melted and combined. Remove from the heat and remove the bowl from the pan, then stir in the soaked chestnuts and rum and the egg yolks. Using a large metal spoon, fold in the ground almonds and meringue.

Spoon the cake mixture into the prepared baking tin and level the surface. Bake in the oven for 18–20 minutes or until well risen but still soft and a skewer inserted into the centre of the cake comes out clean. Remove from the oven to a wire rack and leave the cake to cool completely in the tin. The cake will sink slightly, so use the back of a spoon to gently flatten it and even it out. Once cool, refrigerate the cake base (leaving it in the tin), while you make the mousse.

To prepare the bitter chocolate and chestnut mousse, soak the gelatine in cold water until it has softened. Squeeze the gelatine gently to remove excess water, then place the gelatine in a small heatproof bowl with the measured cold water. Place the bowl over a small pan of simmering water and stir the gelatine until it has dissolved, then remove from the heat. Meanwhile, melt the chocolate in a separate heatproof mixing bowl set over a pan of simmering water (as above). While the chocolate is melting, place the chestnut purée, egg yolks and rum in another mixing bowl and use a wooden spoon to combine. Add the dissolved gelatine and melted chocolate to the chestnut purée mixture and stir to mix.

In a separate bowl, whisk the egg whites until foaming, then add the sugar and whisk together until the mixture forms soft peaks. Whip the cream in another bowl until it forms soft peaks. Fold the meringue and whipped cream into the chocolate mixture using a metal spoon, until well combined, then spoon the mousse evenly over the chilled cake base in the tin and level the surface. Refrigerate for about 6 hours or until completely set, before glazing.

Meanwhile, make the candied chestnuts. Line a baking tray with non-stick baking paper and set aside. Cut the chestnuts into quarters. Melt the butter in a non-stick saucepan and once it starts to foam, add the sugar and chestnuts and heat gently, stirring to dissolve the sugar. Increase the heat and boil the mixture until it turns a golden brown caramel colour and is thick and sticky, stirring regularly (stirring will help to encourage crystallisation and a fudge-like texture). Pour the candied crystallised chestnuts on to the prepared baking tray. Leave to set and cool completely at room temperature, then break the candied chestnuts into smaller pieces and store in an airtight container until needed (see Cook’s Notes).

Make the chocolate glaze. Soak the gelatine in cold water until it has softened. Place the sugar, water and cream in a small saucepan and heat gently, stirring to dissolve the sugar, then bring to the boil over a high heat and boil rapidly for 2 minutes. Stir in the cocoa powder and boil over a high heat for a further 1 minute. Remove from the heat and set aside to cool at room temperature, until the glaze is considerably thickened and nearly set, then pour it evenly over the set mousse. Return the cake to the fridge for about 30 minutes to allow the glaze to set completely.

To serve, cut the cake into slices using a warm, sharp knife, then carefully remove from the tin. Place a slice on to each serving plate and decorate with the candied chestnuts. Serve immediately with cream or crème fraîche.

Cook’s Notes

The candied chestnuts can be made up to 3 days in advance. Store them in an airtight container in a cool, dry cupboard.

Any leftover truffle cake can be stored in an airtight container in the fridge for up to 3 days.