Prep: Preheat the oven to 350°F. Line a 12x17" baking tray with parchment paper only on the bottom. Set aside.
Separate eggs: Separate the egg yolks from the egg whites into two separate large mixing bowls, being careful not to get any yolk in the egg whites. Set the bowl of egg yolks aside.
Beat egg whites: To the bowl of egg whites, add cream of tartar. Use an electric hand mixer to beat the egg whites on medium speed until they become foamy like capuccinno foam.
Add sugar: Sprinkle in 100g of sugar, one tablespoonful at a time, mixing well in between each addition.
Whip to stiff peaks: After all the sugar has been added, keep beating until the meringue becomes stiff peaks. When you pull the mixer out of the bowl, the meringue should stand up on its own in small spikes and the tips should curl over. Set the meringue aside.
Egg yolk mixture: To the bowl of egg yolks, add 50g of sugar, milk, oil, ube extract, and vanilla extract. Use the same hand mixer to mix until combined.
Sift in flour: Use a fine mesh sieve to sift in the flour and mix again until just combined.
Fold meringue into egg yolk mixture: Add about ⅓ of the meringue into the egg yolk mixture. Use a spatula to fold them together until just combined. Add the rest of the meringue and fold until no streaks remain.
Fill pan: Pour the batter into your lined baking tray. Use an offset spatula to push the batter into the corners of the pan and smooth it into an even layer.
Bake: Bake the cake for about 12-13 minutes or until the edges of the cake are slightly golden brown.
Release the cake: Let the cake cool in the pan for about 5 minutes. Don't let it cool completely. The next steps should be done while the cake is still warm. Run an offset spatula around the edges of the cake to release it from the pan.
First flip: Place a wire rack on top of the baking tray. Grab both sides of the tray and the rack and flip them upside down. Remove the tray and the cake will be on the rack. Peel the parchment paper off the cake.
Second flip: Place a new sheet of parchment paper on top of the cake. Grab both sides of the rack and the parchment paper and flip them upside down again. Remove the rack. The nicest side of the cake sheet will now be facing down on the parchment paper and will become the outside of the cake roll.
Pre-roll the cake: Starting at one short end of the cake sheet, roll the cake up with the parchment paper. Let it rest in this rolled-up log shape until it cools completely.