Preheat oven to 170 C / 150 C fan and grease and line a Swiss roll tin.
Whisk together egg yolks, 15 grams sugar and the vegetable oil. Mix to combine and set 2 tablespoons of this batter aside.
Add the cocoa powder to the remaining batter and set to one side.
In a bowl, whisk 1 egg white until foamy then slowly add 2 teaspoons of sugar until you have stiff peaks. Add in the 2 tablespoons of batter you set aside and 1 teaspoon flour, and mix.
Put this mixture into a piping bag, and trace around a giraffe print template to create the outline. Bake in the oven for 1 minute then remove.
Meanwhile, whisk 3 egg whites until foamy then slowly add 50 grams caster sugar until you have stiff peaks. Take this mixture and fold it into the cocoa batter you set aside earlier. Fold in the remaining flour, ensuring there are no pockets of flour left. Be care not to knock too much of the air out.
Pour this mixture over top of your cooked white batter and spread it evenly into the corners of your tray. Bake for 12-14 minutes or until springy to the touch then remove from the oven and leave to cool for 3 minutes.
Remove the cake from the tin and, whilst hot, roll it up, from the short side, with the pattern facing outwards. Doing this makes it easier to roll later and prevents the sponge cracking. Leave to cool completely.
Once the cake has cooled, unroll and spread with the milk chocolate ganache, roll up again into a log and serve straight away.