Preheat your oven to 175C / 350F (fan off), 155C / 310F (fan on). For this recipe I used a jelly roll pan measuring 37 x 24cm / 14.5 x 9.5-inches. Spray the bottom only with oil spray and line with baking paper. Set aside.
Combine dry ingredients - To a small mixing bowl, add the flour, corn starch, baking powder and salt. Whisk to combine. Set aside.
Whip meringue - To a separate medium sized mixing bowl, add the egg whites and cream of tartar and beat on medium speed using an electric hand mixer until you reach soft peaks, about 1 minute. Begin slowly adding (100g) of the caster sugar, about 1 tsp at a time, while the mixer is on high speed. The meringue will thicken and become glossy. Continue beating for 2-3 minutes until the sugar is mostly dissolved. Set aside.
Whip egg yolks and sugar - Add the egg yolks, warm water, remaining sugar (60g) and vanilla extract to a separate large bowl and whip on high speed for 5 minutes until it becomes thick and pale. Then, while the mixer is on high speed slowly drizzle the oil in until well combined.
Fold meringue through the egg yolks - Add half of the egg white mixture to the egg yolk mixture and use a spatula to gently fold through until almost completely combined. Add the remaining egg white mixture and repeat until mixture is uniform.
Add dry ingredients - Sift half the dry ingredients over the egg mixture. Gently fold through until well combined. Sift the remaining dry ingredients in and repeat. The batter will be thick, airy and light.
Bake - Add the batter to the prepared baking tray and use a spatula to gently spread the mixture as evenly as you can. Bake for 14 minutes.
Cooling down - As soon as the sponge comes out of the oven, cover the pan with foil and seal the edges to trap the steam. This keeps the sponge soft and flexible, making it easier to roll without cracking. Let it cool at room temperature for one hour. Just above room temperature is fine, but don’t cool it in the fridge or a cold room or it will stiffen and crack when rolled.