Preheat oven to 180°C (350°F) conventional (see note 2 if using a convection oven with a fan) and lightly grease only the bottom of a 9x13-inch cake pan. Leave the sides of the pan ungreased and do not line them (see note 3).
Sift together flour, cornstarch, cocoa powder, instant coffee powder, baking powder and salt. Mix until well combined and set aside.
In a medium sized bowl add in egg whites and using a hand or stand mixer, whip on a medium-high speed for 30 seconds until foamy (see note 4). Once foamy, gradually add in ½ cup (100g) of the sugar and then continue mixing until you reach stiff peaks (this should take about 4-6 minutes). Set aside.
In a large bowl, combine the remaining ½ cup (100g) of sugar with the warm water. Stir to coat the sugar. Add the egg yolks and vanilla, then whip on medium-high speed for about 2 minutes, until the yolks are pale and slightly thickened.
While mixing on medium speed, slowly stream in the vegetable oil. Mix until well combined.
Fold in half of the whipped egg whites into the yolk mixture using a spatula, being careful not to overmix. Next, gently fold in the sifted dry ingredients. Finally, fold in the remaining egg whites until just combined. If there are large chunks of egg white, gently break them up by pressing them against the side of the bowl with your spatula—but avoid overmixing and deflating the batter.
Pour the batter into the prepared cake pan and spread it out evenly. Run a toothpick or thin knife through the batter to pop any large air bubbles. Lightly tap the cake tin on the counter twice to release excess air, then bake for 25 minutes. The cake is done when the top springs back slowly after a gentle touch.
Once baked, immediately drop the cake tin from a height of about 10cm onto your counter to help release steam. Then, invert the cake tin (with the cake still inside) onto a wire rack and let it cool for 30 minutes (see note 5).