Preheat oven to 180 C / Gas 4. Line a 23x30cm (9x12 in) Swiss roll tin with baking parchment.
For the sponge:
In a small bowl, whisk the egg whites until very stiff.
In a separate bowl, whisk the yolks and sugar together until they form a smooth cream. Sift in the cocoa powder and cornflour and gently fold in the egg whites.
Transfer the sponge mixture to the prepared Swiss roll tin and spread smoothly and evenly.
Bake in the preheated oven for 12 to 15 minutes until a skewer inserted into the centre comes out clean.
Remove from the oven and carefully tip the sponge on to a clean wet tea towel, remove the parchment and quickly roll the sponge up in the towel and leave until cool for about 20 minutes.
For the cream filling:
In a bowl, whisk together the double cream and 1 tablespoon icing sugar until thick. Set aside.
For the chocolate buttercream:
In a bowl, beat the butter together with the icing sugar and cocoa powder until fluffy. Set aside.
Once the sponge has cooled, very gently unroll (do not let it lay flat otherwise it may break) and carefully spread the cream filling onto the sponge. Gently re-roll and transfer to a serving plate. Using a spoon, cover the log with the chocolate buttercream and use a fork to add some log-like texture.
Slice and serve.
Gluten free ingredients
To be sure this recipe is 100% gluten free, if needed, check the labels of all ingredients, especially the cornflour, which can sometimes be contaminated with gluten.