Heat oven to 200C/fan 180C/gas 6.
Grease and line a 23 x 32cm Swiss roll tin
with baking parchment. Beat the eggs and
sugar together with an electric whisk for
about 8 mins until thick and creamy.
Mix the flour, cocoa and baking powder
together, then sift onto the egg mixture. Fold
in very carefully, then pour into the tin. Now
tip the tin from side to side to spread the
mixture into the corners. Bake for 10 mins.
Lay a sheet of baking parchment on
the work surface. When the cake is ready, tip
it onto the parchment, peel off the lining
paper, then roll the cake up from its longest
edge with the paper inside. Leave to cool.
To make the icing, melt the butter and
chocolate together in a bowl over a pan of
hot water. Take from the heat and stir in the
syrup and 5 tbsp cream. Beat in the icing
sugar until smooth. Whisk the remaining
cream until it holds its shape. Unravel the
cake, spread the cream over the top, scatter
over the crushed mints, if using, then
carefully roll up again into a log.
Cut a thick diagonal slice from one
end of the log. Lift the log on to a plate, then
arrange the slice on the side with the diagonal
cut against the cake to make a branch.
Spread the icing over the log and
branch (don’t cover the ends), then use a
fork to mark the icing to give the effect of
tree bark. Scatter with unsifted icing sugar
to resemble snow, and decorate with holly.