Prehear oven to 200celsius. Line and grease a swiss roll pan.
Beat egg whites first till foamy and gradually add in sugar. Whip till soft peaks form.
Put all the yolks into the egg white mixture and beat at medium speed for 3 minutes. Fold in flour and vanilla.
Make sure no trace of flour is left. Fold in oil very well till no longer visible.
Pour onto tray and bake for 7-8 minutes, no longer.
Invert onto a cake rack and let cool COMPLETELY. When thoroughly cool, place on a big sheet of parchment or wax paper and spread buttercream onto cake. With the aid of the paper, roll it up jelly roll style.
Roll the top of the roll in the fine castor sugar. Chill for at least 30 mins to ensure it "sets".