Preheat oven to 350℉ and line the bottom of three 6" cake pans with a round of parchment paper. Do not line or grease the sides of the pans. Set aside.
In a large mixing bowl, use an electric hand mixer to beat egg whites and cream of tartar together until it resembles cappuccino foam.
Sprinkle in 100g of granulated sugar one tablespoonful at a time, mixing well in between each addition. After all the sugar has been added, continue beating until it becomes stiff peaks. The meringue should stand up on its own and won't budge when you flip the bowl upside down. Set aside.
In another large mixing bowl, use the same electric mixer or whisk to mix the egg yolks and the other 20g of granulated sugar together until combined. Add the milk, oil, vanilla extract, pandan extract, and green gel food colouring (optional) and mix again until combined.
In a small bowl, stir together the flour and baking powder. Sift this mixture into the egg yolk mixture. Fold together with a spatula, smushing down any big lumps, until combined.
Add ⅓ of the egg white meringue to the egg yolk mixture. Gently fold together with a spatula until mostly combined. Add the rest of the meringue and fold until no streaks remain and the batter is fluffy and airy.
Divide the batter into your lined cake pans until about ⅔ full, about 200g of batter in each pan. Bake for 23-25 minutes or until the tops are lightly browned and the cakes have some resistance when lightly pressed.
Let the cakes cool on a wire rack upside down for about 30 minutes. Run an offset spatula around the edges and release the cakes onto the wire rack to finish cooling completely before assembling the cake.