the next it is undeniably a thick custard. Cornstarch is neat like that.) Transfer the custard to a bowl, cover with plastic wrap or a round of wax paper, and cool. Then, chill custard, covered, for at least 3 hours, or up to 2 days.
Bake the cakes: (you can also do this step the night before)
Preheat the oven to 325 degrees. Line the bottoms of two 9-inch round cake pans with lightly oiled (I use cooking spray) parchment paper . Otherwise, leave the pans ungreased.
Sift together the flour, 1 1/4 c. sugar, baking powder, and salt twice into a large bowl.
In a separate bowl, use an electric hand mixer on high speed to beat together the yolks, water, oil, zest, and vanilla until smooth. Stir into the flour mixture.
In another large bowl, or the bowl of a stand mixer, beat the egg whites with the cream of tartar until soft peaks form. Add the remaining 1/4 c. sugar, and beat on high until the peaks are stiff but not dry.
Using a rubber spatula (and a very light touch), fold about a quarter of the fluffy egg whites into the egg yolk mixture. Then fold in the remaining whites. Be very gentle. The goal here is to incorporate the egg whites without allowing them to deflate significantly. It is all of the air that has been whipped into the egg whites that will make for tall and light cake layers. As soon as the egg whites are no longer visible, stop folding.
Scrape the batter into the two prepared pans and spread evenly. (Here is a trick for making sure you have poured an equal amount of batter into each pan: Grab two toothpicks and stick one into the center of each batter-filled pan. Then, pull them out and see if the amounts of batter on the picks line up.)