Preheat oven to 350℉ and line a 12x17" baking tray with parchment paper on the bottom, trimmed to fit. Set aside.
Separate the egg whites and egg yolks into two large mixing bowls, being careful not to get any yolk in the egg whites. Set aside the bowl of yolks for now.
To the bowl of egg whites, add cream of tartar. Use a hand mixer fitted with a whisk attachment to beat the egg whites on low-medium speed until it becomes frothy like cappuccino foam.
Sprinkle 100g of granulated sugar into the egg whites, one spoonful at a time, mixing thoroughly between each addition. When all the sugar has been added, continue beating until the meringue reaches stiff peaks. Set aside.
To the bowl of egg yolks, add the remaining 50g of granulated sugar and mix with the same hand mixer until combined. Add the milk, oil, vanilla extract, melon flavouring (optional), and green gel food colouring. Mix again until combined.
Sift in the cake flour and fold together until no big lumps remain.
Add ⅓ of the meringue to the egg yolk mixture. Use a spatula to fold together until combined. Add the rest of the meringue and fold together until no streaks remain and the batter is airy and fluffy.
Pour the batter into your lined baking tray. Use an offset spatula to push the batter into the corners of the tray and smooth the top.
Bake for 12-13 minutes or until the edges are lightly golden brown and the top surface has some resistance when you press on it.
After taking it out of the oven, let the cake cool for a few minutes before running an offset spatula around the edges of the cake sheet to release it from the pan.
Flip the cake out onto a wire rack, peel off the sheet of parchment paper, and then flip it again onto a new sheet of parchment paper so that the side that you want to be on the outside of the cake roll is on the bottom.
Cut the browned edges off the two shorter ends of the cake sheet. Roll the cake sheet up with the parchment paper and let it cool completely resting in this rolled-up log shape.