Preheat your oven to 180 C (350 F). Grease and line a 40cm x 30cm rimmed cookie tray with baking paper.

Place the egg whites into a large, clean bowl and the yolks into a medium bowl.

Whisk the egg whites using electric beaters until you get soft peaks. Gradually whisk in 50g of the sugar until the mixture turns stiff and glossy.

Gradually whisk the remaining 50g of sugar into the yolks using the electric beaters until pale and thick.

Pour the yolk mixture plus the vanilla into the whipped whites and whisk in with the electric beaters just until combined.

Mix some of the beaten egg mixture into the melted butter, then pour this mixture back into the large bowl. Sift the flour and cornflour into the large bowl too. Fold everything together gently using a spatula until just combined.

Pour onto the prepared cookie tray and spread out over the whole tray in an even layer. Bake for 15 minutes - the cake should be evenly golden and crisp at the edges.

Flip the hot cake out onto a clean kitchen towel which has been dusted with icing sugar. Peel off the baking paper and dust the whole cake with more icing sugar. Trim all the edges of the cake so it's a ~28cm square (now eat those edge trimmings!!). Starting at the short edge, *tightly* roll the hot cake up with the towel (so the towel is literally rolled in between the cake layers, not just around the outside). Leave the cake to cool completely.