Put egg whites in a stand mixer bowl and mix until frothy. Add the cream of tartar and mix. Add the sugar intermittently three times and mix until stiff peaks.
Add the cream of tartar and mix. Add the sugar intermittently -3 times and mix until stiff peaks.
You can tell if it is stiff peaks when meringue forms a horn when you pull the whisk upward.
When the meringue is ready, scoop about a cup of meringue mixture and add it to the egg yolk mixture.
Using a spatula, scoop the mixture, then pull up until all of the mixture is incorporated. Do this gently. Folding too hard will result in a hard chiffon cake. Add the rest of the meringue and gently do the same until there are no white streaks.
Transfer the mixture to the prepared pan and tap the pan on a flat surface to remove air bubbles.
Bake in a pre-heated 350 F oven for 20 minutes. Start checking at fifteen minutes using a toothpick test method. The batter will triple in size, and the top should be light golden brown. Push the middle of the cake, and if it springs back, the color is golden, and when the toothpick comes out clean, the cake is done.