In a very large, completely clean and dry bowl or bowl of a stand mixer, use an electric mixer or the whisk attachment and start mixing the egg whites. After about 30 seconds, add the cream of tartar and keep mixing until the egg whites are starting to thicken and turn frothy. See post for visual.
Start adding the sifted granulated (or castor) sugar, 2 tablespoons at a time, and mix for a full 20 seconds between each addition.
Once all of the sugar is added, keep mixing the meringue until medium-stiff, glossy peaks form. When you lift the whisk and flip it upside down there should be a peak that sticks straight up with a slight curl at the tip.
Use a large balloon whisk to very gently mix the meringue into the pandan cake batter. Be careful not to deflate the meringue and mix just until the batter is evenly combined. Use a rubber spatula to wipe around the bottom and sides of the bowl.
Pour the batter into an ungreased 10-inch tube pan. Make sure the pan is not non-stick and don't grease the pan or line it with parchment paper or the cake won't work.
Bake the cake for 55-60 minutes. It should have risen up with a golden brown, crackly top. A cake tester or toothpick should come out clean.
Take the pan out of the oven and very carefully flip the pan upside down. Since you didn't grease the pan and the pan is not non-stick, the cake will stick to the sides and won't fall out. Keep the pan upside down and let the cake cool completely at room temperature. Cooling the cake upside down prevents it from deflating.
Once the cake has cooled, run a knife around the edges of the pan to remove the sides.