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.