1.Preheat oven to 160°C. Grease two deep 20cm square cake pans; line bases and sides with baking paper, extending the paper 5cm over the sides.
2.Stir chocolate, cocoa powder and the water in a medium saucepan over low heat until smooth. Transfer mixture to a large bowl; cool 15 minutes.
3.Add butter, eggs, sugar, extract, sour cream and sifted flours and soda to chocolate mixture; beat with an electric mixer on low speed until combined. Increase speed to medium; beat for 2 minutes or until mixture is smooth and changed to a paler colour. Spread mixture into pans.
4.Bake cakes about 1 1/2 hours. Leave cakes in pans for 5 minutes, before turning, top-side up, onto wire racks to cool.
5.Make chestnut cream: Beat cream in a small bowl with electric mixer until soft peaks form. Whisk chestnut puree in a medium bowl until smooth; fold cream into chestnut puree. Refrigerate until required.
6.Split cold cakes in half. Place one cake layer on a serving plate; spread with about one-fifth of the chestnut cream. Top with another cake layer. Repeat layering, finishing with a cake layer. Spread remaining cream all over cake.
7.Press nuts onto sides of cake. Using a vegetable peeler, make curls from the extra chocolate; sprinkle on top of cake.
Make sure the butter and eggs are at room temperature before starting this recipe. The decorated cake will keep in an airtight container, in the fridge, for up to 3 days. Freeze unfilled cakes for up to 3 months.Note