Preheat oven at 180ºC. Grease and line a 17 X 12 inch aluminium tray with butter paper.
Place castor sugar, egg yolks, eggs and vanilla essence in a food processor. Process till ribbon stage is achieved. Transfer into a bowl.
Mix together refined flour and cornstarch in another bowl.
Sift the flour mixture into the egg mixture and fold lightly till well blended.
Take egg whites in another bowl. Add cream of tartar and beat till soft peaks are formed.
Add beaten egg whites to the egg-flour mixture and fold lightly.
Pour the batter in the prepared tray and level it out with a palate knife. Put tray in preheated oven and bake for 6-8 minutes.
Spread a kitchen towel on a worktop. Dust powdered sugar on it, upturn the tray on it and remove the butter paper. Dust with some more powdered sugar on top of the cake, roll along with the towel and set aside for 10-15 minutes or till the sponge cools.
Open the sponge roll, spread mixed fruit jam evenly all over the sponge and roll without the cloth.
Cut into slices and serve.