Preheat your oven to 160°C (320°F) with the fan on. If your oven does not have a fan function, see note 2. Grease or line only the bottoms of three 8-inch cake tins with baking paper circles; leave the sides ungreased. In a bowl, sift together the flour, cornstarch, cocoa powder, instant coffee powder, and salt. Whisk the dry ingredients to combine and set aside.
In a small bowl, mix the vegetable oil and melted butter, then set aside. In a large heatproof bowl, crack in the eggs and add the sugar and vanilla extract. Whisk together and place the bowl over a pot of simmering water, ensuring the bowl does not touch the water. Stir continuously until the mixture is warm to the touch and the sugar has completely dissolved—this can be checked by rubbing a bit between your fingers or with a thermometer reaching 45°C (113°F).
Remove the bowl from the heat immediately to avoid cooking the eggs. Using a hand or stand mixer on medium-high speed, whip the egg mixture until it triples in volume and is thick enough to form ribbons when lifted with a spatula—this usually takes 5 to 10 minutes. Reduce the mixer speed to low and continue mixing for another 2 minutes to stabilize the batter and remove any large air bubbles.
Sift half of the dry ingredients over the whipped eggs and gently fold in with a spatula until almost combined. Repeat with the remaining dry ingredients, folding just until combined. Next, fold in the oil and butter mixture gently. If the butter has solidified, warm it slightly before adding.
Divide the batter evenly among the three cake tins. To remove large air bubbles, run a toothpick through the batter and gently tap the tins on the countertop. Bake for 20-22 minutes, or until the tops are set and a light press creates an indent that slowly bounces back.
Once baked, drop each cake tin from about 10 cm height to release steam, then invert the cakes to cool upside down for 30 minutes. Afterward, run a thin knife around the edges to loosen the cakes and turn them onto a wire rack to finish cooling. When completely cool, gently brush off the thin crust on top of each cake layer by hand or with a serrated knife to help the layers absorb the syrup better.