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Line a 35 x 25cm Swiss roll tin with baking parchment and heat the oven to 180C/160C fan/gas 4. Beat the egg whites with an electric whisk until just holding peaks, then add half the sugar, 1 tbsp at a time, until the peaks hold. Set aside. Beat the yolks with the remaining sugar until pale and fluffy in a clean bowl. Sift in the cocoa, spices and a pinch of salt and gently fold them through. Beat in a third of the egg whites, then gently fold through the rest until you have an airy mousse. Pour into the tin and spread evenly. Bake for 12-15 mins until springy.
Sprinkle caster sugar over a large sheet of baking parchment. Turn the cake out onto the sugared parchment, then peel off the sheet you used to line the tin. Cover with a clean tea towel, then leave to cool completely. Trim the edges, then score along the inside edge of one of the short sides and roll it up from there, using the parchment to help.
To make the filling, beat the butter, icing sugar, brandy and vanilla together with an electric whisk for 10 mins until light and fluffy. Unroll the sponge, then turn it so that one long edge is facing you. Spread the buttercream over the sponge using a palette knife, keeping the end furthest from you clean. Using the parchment, roll the sponge up again into a roulade.
For the frosting, tip the chocolate into a large bowl. Put the cream and butter in a saucepan and heat gently until the butter melts and the cream reaches simmering point, then pour over the chocolate. Stir to create a smooth mixture, then leave to cool, stirring occasionally, until you can spread it over the sponge. Make bark lines using a fork, then chill in the fridge for at least 30 mins.
To decorate with chocolate leaves, melt the white chocolate in a bowl over simmering water and paint the underside of the bay leaves with it. Put in the fridge to set for 30 mins, then gently peel away the bay leaves. Arrange over the log, then lightly dust with the icing sugar and lustre.