Prep: Raise the oven temperature to 350°F. Clean off your lined baking tray from toasting the coconut flakes and set aside.
Divide eggs: Separate the egg whites and egg yolks into two separate large mixing bowls, being careful not to get any of the yolks into the egg whites. Set the bowl of egg yolks aside.
Egg white meringue: To the egg whites, add cream of tartar. Use an electric hand mixer to beat the egg whites on low-medium speed until they become frothy like cappuccino foam. Add 100g of granulated sugar, one spoonful at a time, mixing well between each addition. After all the sugar has been added, continue beating the egg whites on medium-high speed until they become stiff peaks. Set aside.
Egg yolk mixture: To the egg yolks, add the remaining 50g of sugar and use the same hand mixer to mix until combined. Add the milk, oil, pandan extract, and green gel food colouring. Mix until combined.
Sift in flour: Use a fine mesh sieve to sift the flour into the egg yolk mixture. Mix until just combined and smooth.
Combine egg whites and egg yolks: Add about ⅓ of the egg white meringue into the egg yolk mixture. Use a spatula to fold them together until combined, making sure to scrape the bottom of the bowl as that's where thick batter accumulates. Add the rest of the meringue and fold together until no streaks remain.
Fill pan: Pour the batter into your lined baking tray. Use an offset spatula to spread the batter into the edges and sides of the pan and smooth the top into an even layer.
Bake: Bake for about 12-13 minutes or until the edges are lightly browned and the top is puffy. Let the cake cool in the pan for only 5 minutes before running an offset spatula around the edges to release the cake from the pan.
Release cake: Place a wire rack (or parchment) on top of the pan, grip both sides of the pan and rack (with oven mitts), and flip the cake onto the rack. Peel the parchment paper off the cake. Place a fresh sheet of parchment paper on top and flip the cake onto the parchment paper.
Pre-roll the cake: Use a serrated knife to cut the browned edges off the two short ends of the cake sheet (optional). While it's still warm, roll up the cake sheet with the parchment paper, starting from one short end of the sheet. Let the cake cool completely in this rolled-up log shape.