Place the sugar and egg yolks into a bowl, mix using the whisk on a hand mixer until the mixture turns a paler colour. Stir in the chestnut puree, butter and nuts and beat the egg whites with the salt until stiff. Stir in 1/3 of the egg whites and carefully fold in the remaining mixture using a rubber spatula. Transfer the cake mixture to the prepared tin.
Bake for approx. 15 mins. in the lower half of an oven preheated to 180°C. Remove, scatter with the brittle, finish baking for approx. 45 mins. and allow to cool slightly. Remove the frame of the tin, transfer to a cooling rack and allow to cool.
Coat the sides of the cake with chocolate. Immediately transfer the cake to a clean sheet of baking paper and decorate the sides with the brittle. Dust the cake with icing sugar.