Pre heat the oven to 370°F (190°C) 170° (fan).
Line a 15x10 inch (39 x 27cm) jelly-roll pan with parchment/baking paper.
Egg yolk mixture: Combine oil, cocoa powder and instant coffee granules (if using) in a separate measuring jug or bowl of hot water.
In a separate large bowl, use a hand mixer or a stand mixer fitted with a whisk attachment to beat the egg yolks for 2 minutes. Gradually add sugar and beat for another 2 minutes until pale.
Add the combined cocoa/coffee/oil mixture into the beaten egg yolk mixture and beat for 1 minute.
Sift the flour, baking powder and salt into the egg yolk mixture then whisk or beat on low speed for 1 minute until combined. This will form a thick batter.
Egg whites: Beat egg whites in large bowl until soft peaks form
Combine egg yolk mixture with beaten egg whites: Use a spoon to gently transfer the beaten egg whites into the batter. Fold the beaten egg whites gently with a rubber spatula into the thick batter.
As you fold in the egg whites, you will notice the mixture transform into a creamy, rich batter.
Pour batter into pan and bake: Pour and spread the batter evenly into the lined baking pan.
Bake for 12 to 15 minutes or until top of the cake springs back when touched lightly in center. (Refer to notes below: Tip on Why does the cake crack when rolling?)
Roll cake while still warm: Leave the cake for 1 minute in the pan, then turn the cake out onto a sheet of parchment paper or soft kitchen towel dusted with powdered sugar. (Refer to notes below: I have included an easy tip for you below, if you are finding it difficult to flip out the cake directly from the pan.)
Gently Peel off the parchment paper on which the cake was baked. (Refer to notes below: Roll cake while still warm)
Use the sugar dusted parchment paper or kitchen towel to help you to tightly roll up the sponge from the short end and leave it to cool completely.