Lightly grease a swiss roll tin and cover with baking paper.
Add a little lemon juice to the soya milk and leave to thicken and curdle
Pour the aquafaba into a large bowl and whisk until thick and stiff. Add the cream of tartar and whisk again, then gradually pour in the coconut sugar whisking all the time. Finally add the vanilla essence and soured soya milk, whisk again to keep thick and light.
Pre-heat the oven to 180ºC.
Sift the flour, cocoa powder and baking powder into a separate bowl. Mix well to ensure everything is combined
Carefully pour the flour mix into the aquafaba mix, quickly and lightly folding it in. Try to keep as much of the air present as possible. It will become thick.
Quickly spoon out onto the prepared baking tray and lightly spread out to evenly cover – don’t press it down though. Tap on the work top a couple of times.
Place the tray in the oven and bake for 8 minutes.
Place a clean tea towel on the work top and cover it with a clean piece of baking paper. Sprinkle the caster sugar over the paper.
Check the cake -if the sponge is firm but springy, it’s ready. If it’s still a little wet, return to the oven for another couple of minutes but do not overbake.
Remove the tin from the oven. Carefully turn the cake out onto the prepared baking paper. Peel away the paper from the top, then roll up in the new baking paper lengthways. Transfer to a cooling rack and leave to cool.
Make the chocolate frosting by placing the prepared sweet potato or avocado, maple syrup and cocoa/cacao powder into the bowl of a small food processor. Whizz until the mix is well combined and smooth. Taste and add more maple syrup or cocoa as needed for taste and/or texture.
Place the chestnut puree in a bowl and loosen slightly with the soya milk so it is spreadable.
Once fully cool, unroll. It may crack and break a little, but don’t worry!
Spread 1/3 of the sweet potato icing over the cake and the chestnut puree if using. Carefully roll up lengthways.
Cover the outside with the remaining sweet potato cream, creating a wood effect with a fork.
Transfer to a serving plate or board. Sprinkle icing sugar over the top if using and leave to set in the fridge before serving