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Heat oven to 180C/160C fan/gas 4. Line a roughly 37 x 25cm tin with baking parchment. Melt the chocolate over a pan of barely simmering water. Set aside to cool while you get everything else ready. Whisk the yolks and brown sugar until pale and thick enough to hold a trail. Whisk in the cocoa, chocolate, clementine zest and juice. Whisk whites until stiff, whisk a third into chocolate mixture, then fold in the rest. Transfer to tin, gently spread to corners and bake for 16 mins until firm to touch.
Meanwhile, dust a large sheet of baking parchment with a little cocoa. When the cake is done, turn it out onto the parchment. Don’t remove the paper stuck to the bottom of the cake – just roll the cake up with the paper inside from one of its longer sides. Leave to cool. This can be made a day ahead: wrap it in cling film or keep in an airtight container once cool.
For the frosting, melt the chocolate, cream and icing sugar together in a bowl over a pan of barely simmering water. Cool and chill until spreadable. Or chill for up to 24 hrs, then leave at room temp until spreadable again.
For the clementine cream, whisk the cream, crème fraîche, icing sugar, clementine zest and juice until thick. Chill for up to 24 hrs.
To make the decoration, melt the caster sugar with 100ml water in a small pan and gently bubble. Peel the zest from the clementine with a vegetable peeler. Thinly shred into strips, then drop into the bubbling syrup. Simmer for 2 mins, then turn off and leave for another 2 mins. Scatter a little extra sugar onto a plate. Lift out the shreds with a slotted spoon and toss in the sugar. Set aside to dry.
To assemble, gently unroll the sponge and peel off the paper. Squeeze over juice from final peeled clementine. Thinly spread with a tiny bit of the chocolate frosting, then spread with half to two-thirds of the clementine cream. Carefully roll back up and trim each end. Transfer to a serving plate and cover with the rest of the chocolate frosting, using a round-bladed knife to stripe the surface. Scatter with the sugared clementine peel and silver balls and/or stars, if you like. Serve with remaining clementine cream.