Hooray! A caterpillar cake has come crawling into The Great British Bake Off! There are no ugly bugs at this ball! This is a chocolate Swiss roll evolved into a gorgeous birthday treat… The question is: what will you call yours?
Make the sponge. Heat the oven to 220°C/200°C fan/425°F/Gas 7. Whisk the eggs and sugar in a stand mixer fitted with the whisk, on high speed for 2-3 minutes, until it is pale, thick and leaves a ribbon trail.
Sift the flour and cocoa powder into the bowl and gently fold it in using a large metal spoon, taking care not to beat any of the air out of the mixture.
Pour the mixture into the prepared tin and spread it evenly into the corners with a palette knife. Bake the sponge on the middle shelf for 10-12 minutes, until it is well risen, firm to the touch, and starts to shrink away from the sides of the tin. Remove from the oven and reduce the temperature to 120°C/100°C fan/250°F/Gas ¾.
Place a sheet of baking paper larger than the Swiss roll tin on the work surface and sprinkle with caster sugar. Carefully invert the cake onto the paper and remove the lining paper. Score a line 2cm in from the edge of one of the short sides, making sure you don't cut all the way through the sponge. Starting from the scored short side, tightly roll up the sponge using the paper to help, then transfer it to a wire rack to cool completely.
For the Swiss meringue. Tip the sugar into a clean heatproof bowl with the egg whites and salt. Set the bowl over a pan of gently simmering water and using an electric hand whisk, whisk until the sugar dissolves. Continue whisking for 3-5 minutes until the mixture thickens enough to hold a ribbon trail and the temperature on a sugar thermometer reaches 71°C/160°F.
Spoon the mixture into the bowl of a stand mixer fitted with a whisk attachment, and whisk for 3 minutes until the mixture has doubled in volume and is thick and glossy. Divide half of the meringue between 3 small bowls, leaving the remaining half of the meringue in the mixer bowl.
Working with the meringue in the 3 bowls, colour one bowl green, one bowl red and one bowl yellow. Spoon each coloured meringue into a small piping bag fitted with a small star nozzle. (If you have only one nozzle, then work with one colour at a time, washing the nozzle in between.)
Using the red, yellow and green meringue, pipe 7 small meringue kisses of each colour, about the size of a hazelnut, onto the lined baking sheet. Using the green meringue, pipe six legs in an 'L' shape, measuring about 2 x 4cm, and six legs in a reverse 'L' shape. Pipe two green curved antennae, each about 4cm long. You can use any remaining meringue to pipe flowers to decorate the serving plate or board.
Bake the decorations for 45 minutes, until dry. Remove from the oven and leave to cool on the baking sheet.
Make the ganache coating. Put the chocolate and butter in a heatproof bowl and set aside. Pour the cream into a pan and slowly bring it just to the boil. Pour it into the bowl with the chocolate and butter and leave the hot cream to melt them slightly for 1 minute, then stir until smooth and glossy. Leave the ganache to cool and thicken before using.
Prepare the caterpillar face. Melt the white chocolate in a heatproof bowl set over a pan of gently simmering water, stirring until smooth. Place the 7cm cutter in the middle of the lined plate and pour in the melted chocolate, then leave it to cool and set.
Make the chocolate Swiss meringue buttercream filling. Melt the chocolate in a heatproof bowl, set over a pan of gently simmering water. Stir until melted, then remove from the heat and leave to cool.
Gradually add the butter to the meringue in the bowl of the stand mixer, whisking continuously until smooth. Fold in the cooled, melted chocolate. Chill until ready to use.
Assemble the caterpillar. Unroll the cooled Swiss roll and remove the baking paper. With one of the short sides facing you, make thirteen vertical cuts, 1.5cm apart and 15cm long into the sponge, so you now have fourteen strips of sponge that look like the keys on a piano. Cut the top of each alternate strip to remove seven sponge fingers (blitz or crumble these to make 'soil').
Spread the filling over the sponge and re-roll the Swiss roll tightly from the other (non-cut) short end (the strips will make the ridges on the caterpillar body). Transfer the sponge to a wire rack set over a sheet of baking paper. Carefully pour the thickened ganache over the sponge roll to coat it smoothly, then gently transfer it to a serving plate or board. Arrange the coloured meringue kisses on the ridges of the caterpillar's body, then set the legs and antennae in place.
Decorate the face. Mix the icing sugar with a few drops of water and the black food-colouring paste to make a thick icing, then spoon it into the small piping bag fitted with the writing nozzle. Remove the set white chocolate disc from the mould.
Dust the work surface with cornflour and roll out the red and white fondants until 3mm thick. Cut out two white ovals for the eyes and roll two tiny white balls to add to the pupils. Using the red fondant, cut out two circles for the cheeks and a small tongue shape. Dampen the back of the fondant eyes with a little water and press them in place on the white-chocolate face. Using the black icing, pipe a pupil onto each eye, then pipe a mouth. Dampen the fondant cheeks and tongue and stick them in place on the face. Press a white fondant ball onto each pupil.
Press the caterpillar face onto the thick end of the Swiss roll - the ganache will help it stay in place. Arrange the meringue flowers and chocolate soil on the serving plate or board, to finish.