Preheat your oven to 170C / 340F (fan off), 150C / 300F (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 medium sized mixing bowl, sieve together the flour, cocoa powder, baking powder and salt. Add the instant coffee powder and whisk to combine. Set aside.
Whip meringue - To a separate large mixing bowl, add the egg whites and beat on medium speed using an electric hand mixer until you reach soft peaks, about 1 minute. Begin slowly adding half (70g) of the caster sugar, about 1 tbsp at a time, while the mixer is on high speed. The mixture will thicken and become glossy. Continue beating for 2-3 minutes. Set aside.
Whip egg yolks and sugar - Add the egg yolks, remaining 1/3 cup (70g) sugar and vanilla extract to a separate large bowl and whip on high speed for 5 minutes until it becomes thick and pale.
Fold egg yolks through meringue - 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 the dry ingredients for a second time over the egg mixture. Gently fold through until well combined. Lastly, drizzle the melted and cooled butter over the cake batter and fold through until you can see no streams of shiny butter. This batter is very 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 10 minutes.
To take out of the baking tray - Take out of the oven and gently run a knife around the side of the tray to help loosen the cake. Dust with about 2 tbsp of cocoa powder. Place a large piece of baking paper over the cake followed by a large chopping board. Flip the cake over onto the chopping board and take the baking tray away. Peel the baking paper away from the bottom of the cake.
How to cool the cake down - Tightly roll the cake with the baking paper starting from the narrow end. You will have baking paper rolled up with the cake. This is going to help stop the cake from sticking onto itself as it cools and help it retain some rolling memory which will stop it from cracking. Allow to cool completely at room temperature.