Combine sugar and cornstarch
Clean and dry the whisk thoroughly, and in a clean bowl, whisk the egg whites until foamy.
Add half the sugar-cornstarch mixture and continue beating for a few minutes, then add remaining sugar-cornstarch mixture and beat until egg whites are glossy and stiff peak form.
Add one-third of meringue into egg yolk mixture from Part A.
Fold in lightly with a spatula to loosen the mixture a little. Then add the rest of the meringue and fold to incorporate completely.
Pour batter into the prepared pan, spreading the mixture evenly and smoothing the top with a spatula. Lightly tap the bottom of the tray on the work surface.
Bake for 14 minutes until the cake has risen slightly and feels springy to the touch. Remove from oven. Turn sponge straight from oven onto a sheet of baking paper.
Peel off baking paper from the bottom of the sponge. Cover with baking paper, then a damp tea towel. Set aside to cool completely.
You now have your sponge base to work with as you like.