Preheat the oven to 180 degrees Celsius / 350 Fahrenheit.
Separate the egg whites from the yolks, and using an electric mixter, beat the whites with the pinch of salt until it forms soft peaks
Add the caster sugar and continue to beat until it forms stiff peaks and it’s has a shiny meringue-like consistency.
Add the egg yolks one by one and mix well.
No need to use a mixer from now on, a silicone spatula would do just fine.
In go the flour, oil, cocoa powder, baking powder and vanilla extract.
Fold in the ingredients gently, making sure the batter is smooth and lump free.
Line a rectangle baking tray with non-stick baking paper and spread the batter evenly.
Bake for about 12 minutes until the sponge is well risen.
Remove the sponge from the oven, and leave it to cool for 5 minutes.
Carefully roll it into a sausage, and cover it with a clean tea towel, then leave it to cool completely.