Separate 4 egg yolks and 1 large egg.
In another bowl, whisk together 60g corn oil,� 60ml milk, 1/8tsp salt, 20g caster sugar. Sieve 100g flour and 10g corn flour and mix together. Add the egg earlier and mix well.
In another bowl, mix 1tsp of cocoa powder and 2tsp of hot water until thicken.
Whisk 4 egg whites and add in 80g caster sugar in 3 batches. continue to whisk until stiff peaks form.� Mix 1/3 of the egg white mixture with the batter and pour it into egg white mixture and slowly mix well.
Pour 200g mixture into a mixing bowl, add cocoa powder mixture and mix well.
Pour the remaining mixture into a baking tray and smoothen the surface. Pour the chocolate mixture and smoothen the surface. Tap the baking tray a few times to release the air bubbles.
Bake in a preheated oven at 140�C for 20 minutes. Change to 160�C bake for 10 minutes, then 180�C bake for another 10 minutes. Set aside and leave it to cool.
In a pot, add 70g caster sugar, 50ml water and a slice of lemon. stir in medium heat and then change to low heat and continue to boil for 10 minutes. Set aside and leave it cool.
In another bowl, beat 150g butter. Add the lemon sugar mixture and continue to beat until well.
Use a sharp knife to trim the side of the cake.
Add the filling and roll the cake. Chill in the fridge for 30 minutes and you�re good!