If you can't find silver Smarties you can cover coloured ones in tin foil. Just make sure guests don't eat them!
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Cover the cake with marzipan and white icing. (See step 5 for more information).
Knead the ready-to-roll icing, then split into three balls. Leave one ball white, and knead a little green colouring into the other two to give two different shades of green. Roll out each ball to about 5mm thick on a work surface lightly dusted with icing sugar. Stamp out about 15 tree shapes using tree cutters, then leave to dry for a few hours or overnight.
Once firm, lift half of the trees onto a cooling rack. Combine the 200g icing sugar and egg white to make an icing, then drizzle it over the trees with a teaspoon. Scatter with edible sparkles and leave to dry again until solid.
Put a little icing on the back of each tree and press the trees around the edge of the cake, overlapping some to give a 3-D effect. Scatter the sweets over the top of the cake to finish. Can be iced up to a week ahead.
To cover a cake with marzipan, first brush the cake
all over with a thin layer of warmed apricot jam.
Dust the work surface with icing sugar, then roll
out the marzipan evenly until you have a 5mm-
1cm thick round, about 40cm across for a 20cm
cake. Lift over the cake, using a rolling pin to help,
then smooth with your hands and trim off the
excess. Leave to dry overnight or for a few hours.
Lightly brush the marzipan all over with cooled,
boiled water. Roll the icing out as you did the
marzipan, then smooth with your hands, trim off
the excess and leave to dry.