Ingredients

Step by step

1. Preheat the oven to 180°C, fan 160°C, gas 4. Grease and base line a 20cm x 30cm Swiss roll tin with baking paper.
2. Using a stand mixer if you have one (or an electric hand whisk otherwise), beat 3 eggs and 125g sugar together in a large bowl for 10 minutes until pale, thick and doubled in volume. Fold in 100g flour, 35g cocoa powder, a pinch of salt and 1 tablespoon of milk then pour into the prepared tin, spreading to the corners with a palette knife. Bake for 12-15 minutes until springy to the touch.
3. Lay a damp tea towel on the work surface and top with a piece of baking paper larger than the sponge. Dust the paper with 1 tablespoon of caster sugar then, using a table knife, loosen the edges of the sponge all the way around and carefully turn out the warm sponge onto the sugar-dusted paper.
4. Peel away the base paper from the sponge then score a line along one of the shorter ends of the sponge, approximately 1cm away from the edge. Start rolling using this incision and use the sugared paper to help you roll it tightly. Wrap in the damp tea towel and leave to cool completely.
5. Repeat with the rest of the ingredients to make a second sponge.
6. To make the buttercream, using an electric whisk, beat the butter and icing sugar together until pale and fluffy. Beat in the vanilla and a good pinch of salt for a further minute. Unroll the sponges and cut in half lengthways so you have 4 long cake sponges in total.
7. Spread all 4 sponges with the buttercream and carefully re-roll one of the sponges beginning at the end with the incision. Take the second and join it where you finished the first, then continue to roll. Repeat with the remaining sponges to make a large very fat Swiss roll. Sit the rolled sponge upright on a flat cake platter – the top should look like the rings on a tree trunk. You could cut off a thin top slice to expose the rings more cleanly.
8. For the bark, place the dark chocolate in a heatproof bowl over a saucepan of barely simmering water, making sure the bowl doesn't touch the water. Heat until melted and smooth. Remove from the heat and set aside for 15 minutes to cool slightly and thicken. Cut a piece of baking paper 20cm x 22cm and spread the melted chocolate over it, to cover the baking paper. Loosely roll up the baking paper and secure with a little tape, transfer to the fridge to set for about 1 hour until firm. Remove from the fridge and unroll the paper – the chocolate will break into shards as you do so. Keep them chilled until ready to use.
9. For the ganache, pour the cream into a saucepan and place over a medium heat. Heat until bubbles appear in the edges of the cream then remove from the heat and add the chocolate. Leave for 5 minutes to melt then gently mix with a spoon until smooth and glossy. Leave to cool and thicken (or pop in the fridge for 15-20 minutes to speed this up), then spread generously over the sides of the cake. Reserve a little ganache to secure the mushrooms (1-2 tablespoons).
10. Stick the chocolate shards onto the ganache icing to create a bark effect, before the ganache has fully set.
11. Decorate with meringue mushrooms using the reserved ganache to secure, scatter with the pistachios and red sugar sprinkles.