Grease and line a swiss roll baking tin and preheat the oven to 200°C/180°C.
Separate the eggs, putting the yolks into one large mixing bowl and the whites into another.
Using an electric whisk, beat the whites until they form stiff, fluffy peaks.
Add the sugar and vanilla extract to the yolks and beat with a wooden spoon until well combined. Then sieve in the cocoa powder and beat again to combine.
Add the stiff egg whites to the mixture and gently fold until just combined.
Sieve the flour over the mixture and again fold together until just combined.
Pour the mixture over the lined tin and spread it out so it fills the entire surface of the tin evenly.
Bake for 8-10 minutes or until cooked through.
When fresh out the oven, lay another sheet of baking paper over the top of the tray and carefully flip everything upside down so the sponge is now sitting on the fresh piece of paper. Peel the original piece of baking paper off the top.
Slice a tiny slither away from each edge to make straight lines.
Whilst it's still warm, carefully roll up the sponge with the baking paper inside, starting from one of the shorter edges. Leave the rolled up sponge to cool completely.