Line a 9 x 11 baking sheet pan with parchment paper and preheat the oven to 350
In a mixer, mix the eggs and sugar until combined. Set the pan over a saucepan of simmering water (bowl not touching the water). Heat the egg mixture while whisking constantly until warm to the touch (about 2 minutes)
Remove from heat, add the vanilla and whisk the mix (in a mixer or with a handheld mixer) on medium heat until it is a very thick, foamy mix that has doubled in volume.
Sift the flour and cocoa powder together in a separate bowl. Gently fold in half of the flour/cocoa mix into the egg mixture. Add the rest of the flour and cocoa and fold it through the batter.
PLEASE NOTE - handle the batter as little as possible so as to not deflate it - about 15-20 folds each time maximum.
If you choose to add the oil, Gently mix the oil with about 1 cup of the batter separately. Introduce this back into the batter and fold into the batter (with as little handling as possible)
Immediately transfer the batter into the prepared baking pan, gently pushing the batter to the corners and making sure it is spread evenly.
Bake in the oven for about 15 minutes until golden brown and cooked through.
When the cake is ready let it cool out of the oven for a few minutes.
Turn it out on to a new/clean dish cloth and gently pull away the parchment paper it was baked in, but place it back on the cake.
While it is warm, Roll the cake up using the cloth to secure it. Cover it with the cloth and leave it to cool.
When you're ready to ice it, gently unroll the cake and remove the parchment paper.
Spread the icing on the cake surface and re-roll the cake.
Transfer the cake onto a serving plate, seam side down and dust powdered sugar on top.