Sit the cake on a cake board or serving plate. Brush the top and sides with some of the jam. Roll out the marzipan on an icing-sugar-dusted surface until large enough to cover the top and sides of the cake – use a piece of string to help you measure. Carefully lift your marzipan on top and smooth down the sides of the cake to stick. Trim the bottom, then brush all over again with more jam.
Make sure your surface is really clean, then dust again with a little icing sugar. Knead 850g of the icing until it is very smooth and pliable. As with the marzipan, roll out the icing, then cover the cake and once again trim any excess from the bottom. Smooth out any bumps or marks with the flats of your hands, rubbing the icing to buff it to a shine.
Pull off about a fifth of the remaining icing and knead in enough red food colouring to give a bright colour. Pinch off small amounts and roll between your fingers to make little ‘berries’. Sprinkle some glitter on a saucer and roll the balls in it to coat completely. Shake off any excess glitter and set aside to firm up.
Knead some green food colouring into the remaining icing and roll it out thinly on an icing-sugar-dusted surface. Stamp out lots of holly leaves and begin sticking them onto your iced cake. Use a paintbrush to moisten the underside of each leaf with a little water – don’t make them too wet though, or the green colour might leak onto the rest of the cake. When you have added enough holly leaves, mix a splash of water into some icing sugar to make a runny but fairly stiff icing, and use this to stick on your glitter berries. Leave the cake for a good couple of hours to dry before cutting into it. The cake will sit happily uncut for up to 2 weeks. Once cut into, keep in an airtight tin for up to a month.