A caterpillar cake is a classic birthday treat: a chocolate Swiss roll filled with chocolate buttercream, covered with chocolate ganache, decorated with a cute chocolate features and presented to an excited boy or girl on their special day, ideally with candles.
Line the Swiss roll tray with baking paper and preheat the oven to 200C (180C fan, 390f).
Put the eggs and caster sugar in a large mixing bowl. Whisk until thick, pale and considerably increased in volume.
Sift in the flour and cocoa, then fold through gently with a silicon spatula.
Tip the mixture into the lined tin. gently level off and bake for 8 minutes until just firm.
Shape the sponge
Lay a rectangle of baking paper out on your counter and sift the cocoa and icing sugar all over it.
Turn the hot Swiss roll sponge out onto the baking paper and carefully peel off the paper backing.
Lay a clean tea towel on top of the Swiss roll sponge and carefully roll the sponge up with the tea towel and baking paper.
Leave rolled up to cool completely.
Make the frosting
Melt the dark chocolate and set aside to cool.
Put the icing sugar, butter, vanilla and milk in a mixing bowl. Whisk together until pale, smooth and considerably increased in volume.
Add the cooled dark chocolate and whisk to a smooth chocolate frosting.
Fill the Swiss roll
Carefully unroll the sponge and spread with the chocolate buttercream right to the edges.
Roll back up, this time without taking the baking paper with it.
Transfer to a clean piece of baking paper.
Make the ganache
Break all the chocolate into a heatproof bowl and melt.
Add the double cream and whisk together until you have a glossy, thick ganache.
Cover the ganache
Spoon the ganache on top of the Swiss roll and spread all over to cover all sides.
Position the Smarties on top, then leave to set completely - this may take up to an hour.
Make the legs and face
Line a tray with a bit of baking paper.
Melt the white chocolate and transfer to a small piping bag. Snip off the very tip.
Pipe some legs about 2cm/0.75inch tall onto a sheet of baking paper. Caterpillars have six 'true legs', but many species also have a number of 'pro legs', so I decided to give mine four legs on each side.
You'll also need to pipe a face for the caterpillar, aiming to make it about the same width as the end of your Swiss roll. You can pipe the basic shape of the face free hand or, as I did, pour the melted chocolate into a clean Pringles lid.
Use melted milk chocolate in another piping bag to add eye and nose details, then gently add two Smarties to look like rosy cheeks.
Pipe the antennas separately on the tray. Pop the tray in the fridge so that the legs, face and antennae can set.
Once set, cut your caterpillars face free of the Pringles lid mould.
Position the on the front of the cake, the antennae just above, and the legs along the length of the caterpillar's body. Fixing each element in place with a dot of leftover melted chocolate.
Leave to set and then serve.
Keep any leftover caterpillar cake covered in a cool, shaded place or in the fridge. If storing in the fridge, I recommend getting out half an hour before you serve as it tastes better at room temperature.If properly stored in a sealed container in a cool, shaded place or in the fridge, your cake should keep for 3-5 days.In the freezer, leftover slices will keep for up to 3 months.