Preheat the oven to 180ºC/ 350ºF/ Gas Mark 4. Grease a 33 x 23cm (13 x 9 inch) Swiss roll tin.
Cut a sheet of greaseproof paper 4cm (1 ½ inch) larger all round than the tin. Press the paper into the tin, cutting at each corner to ensure the paper fits the tin edges.
In a large bowl, whisk the egg yolks and sugar until very thick and pale. Sieve the cocoa into the mixture and using a large metal spoon, fold the ingredients together.
In another bowl, whisk the egg whites until just stiff but not dry and fold these into the base mixture. Pour into the prepared tin and smooth the surface. Bake in the middle of the oven for about 20 minutes until springy to the touch. Take care not to overcook. Leave to cool in the tin (it will sink on cooling!).
When cool, turn onto a large sheet of greaseproof paper and peel away the lining paper. To make the chocolate butter icing, beat the unsalted butter to soften and gradually sieve and stir in the icing sugar. Add a few drops of Natural Vanilla Extract or the brandy to flavour, stir in the melted Fine Cooks’ Chocolate then spread half over the roulade base.
Taking hold of one end of the parchment, lift it and gently roll the roulade over like a thick Swiss roll, pulling the paper away as you go - don’t worry if the roulade cracks. Gently press together to seal the edge. Cut off a diagonal slice about 5 x 3cm (2 x 1 ¼ inch) thick from one end of the roll. Secure this to one side of the roll using more butter icing to create a log shape.
Transfer to a cake board and cover with remaining butter icing, swirling the icing with a fork to give it a bark-like texture.
Break the Marzipan into 20 small pieces and form half into short stalk-like lengths, and the remaining pieces into flattish ‘cap‘. Press 2 pieces together and stand upright on a plate to form mushrooms. Dust lightly with cocoa powder and set aside.
Roll out the green Ready to Roll Coloured Icing thinly on a surface lightly dusted with icing sugar, and cut out several small leaf shapes and blades of grass. Gently press the leaves on top of the log with a few mushrooms, and arrange the remaining mushrooms around the sides along with the blades of grass. Decorate with Writing Icing and gently dust with icing sugar.