Photo: William Reavell
Pre-heat the oven to 180°C (350°F) Gas 4. For the cake, melt the chocolate in a saucepan with the milk, then once the chocolate is melted, add the flour and whisk until the mixture becomes a smooth paste. Put the egg yolks and caster sugar in a mixing bowl, and whisk together using a mixer or whisk, until the mixture is creamy, pale yellow and has doubled in size. Whisk in the chocolate paste.
Put the egg whites in a clean, dry bowl and whisk until they form stiff peaks, then fold them into the chocolate mixture. Spoon the mixture into the prepared tin using a spatula. Bake for 15–20 minutes until the cake springs back when pressed lightly with your fingertips. Remove the cake from the oven and leave for a few minutes.
Put a sheet of baking parchment on a clean work surface. Invert the cake on to the parchment, cover with a clean damp kitchen towel and leave to cool for 10 minutes. Remove the kitchen towel and lining parchment. Roll up the cake from the short end, using the baking parchment to help you, and leave to cool completely.
For the filling, mix the chestnut purée and two tablespoons of the icing sugar together in a bowl until smooth. Add the cream, and whisk to stiff peaks. Unroll the cake. Spread about two-thirds of the cream in an even layer over the whole sponge. Reroll the cake up tightly from one of the short ends, and place on a serving plate. Cut off one end of the cake at a diagonal angle and place it to the side to make a branch.
For the icing, stir half the cooled melted chocolate into the remaining chestnut cream and add the remaining icing sugar. Whisk until the icing holds stiff peaks. Spread over the whole cake, but leaving the ends uniced. Spread the rest of the cooled melted chocolate out thinly on a silicon mat, and allow to set. Once set, break the chocolate into thin shards. Decorate the log with the shards of chocolate. Dust with icing sugar and add sugar paste decorations. Chill for 3 hours in the fridge before serving. Best eaten immediately, or refrigerate for up to 2 days.