Preparing your oven - Preheat your oven to 165C / 330F (fan off), 145C / 300F (fan on). I like baking with the fan off for an even rise. Position your rack in the middle of the oven.
Preparing your cake tin - Rub cold butter on the bottom (not sides) of a 22x33cm / 9x13-inch cake tin. You don’t want any butter on the sides of your tin because you want the sponge to stick so it holds its shape as it cools instead of shrinking away.
Combine dry ingredients - Run the flour, corn flour, baking powder and salt through a sieve into a large mixing bowl. Use a whisk to combine. Set aside.
IMPORTANT! To clean your bowl for the meringue - To ensure your eggs whip up properly add 1/2 tsp of lemon juice or white vinegar around a large glass or metal mixing bowl and use a paper towel to wipe dry. This removes any grease in your bowl, so your egg whites whip properly. Avoid plastic bowls as they hold onto grease.
Make the meringue - Add the egg whites and cream of tartare to your mixing bowl. Use an electric hand mixer or a stand mixer fitted with a whisk attachment to whisk on medium high speed for 1 minute until frothy. Then, while the mixer is still running, slowly add the sugar 1 tbsp at a time. This will allow the sugar to dissolve properly and should take about 3 minutes for all of it to be added. Continue whisking until the meringue reaches stiff peaks, about 8-10 minutes (usually 8). The meringue should hold it’s shape completely when the whisk is lifted up.
Add water, egg yolks and oil to dry ingredients - Add the water, vegetable oil and egg yolks to the bowl with the dry ingredients and use the whisk to combine until smooth and no lumps are showing. You want to do this when the meringue is almost done whipping.
Fold meringue into batter - Add 1/3 of the meringue into the egg yolk mixture and use a balloon whisk or spatula to fold through until the batter is uniform. No meringue streaks are showing. Then repeat until all the meringue has been added. You will have a thick, fluffy batter.
Bake - Gently pour the batter into the cake tin. Use a spatula or spoon to spread evenly. Bake in the middle rack of your oven for 25-27 minutes or until lightly golden on top. Once it’s baked, let it cool in the tin to room temperature.
Taking out of the tin - Gently run a knife around the inside of the cake tin to loosen it. Place a large cutting board on top and flip over so the cake comes out on the board. Use one hand to hold the top of the cake against the board as you flip it right side up (baking paper on the bottom), onto your workbench.
To trim the cake - Use a large, serrated knife or cake leveler to level the cake to 2cm in height. Use a ruler to cut out 9cm wide cake strips. You’ll have 3 cake layers all up. Excess cake: It might seem like there’s a lot of excess cake left over but less cake would be much harder to trim. Discard the excess cake or turn it into a single serve trifle with cream, fruit and jelly!