Preheat the oven to 375°F Place paper cupcake inserts inside cupcake pan.
Sift the flour, baking powder, and half the sugar together into a large mixing bowl or stand mixer bowl and reserve.
In another large mixing bowl or stand mixer bowl, combine the egg yolks, oil, water, vanilla, and zest. Mix with a handheld mixer or whip attachment until thoroughly combined, about 1 minute.
Add the egg yolk mixture gradually to the dry ingredients, mixing with a handheld mixer or whip attachment on medium speed until a paste forms. When a paste has formed, scrape down the sides of the bowl, and continue adding the remainder of the yolk mixture until it is all incorporated. Beat for an additional 2 minutes on medium speed.
In a separate mixing bowl or stand mixer bowl, whip the egg whites with a clean whip attachment on medium speed until soft peaks form. Gradually add the remaining sugar while beating the egg whites and continue to beat until medium peaks form.
Gently blend 1/3 of the beaten egg whites into the egg yolk mixture to lighten it. Gently fold in the remaining egg whites.
Divide the batter evenly among the prepared cups.
Bake at 375°F until the top of a cake springs back to the touch, about 20 minutes.
Let the cakes cool in the pans for a few minutes before removing them to a wire rack to finish cooling. The cakes are ready to frost now, or they can be wrapped and stored at room temperature for 2 days, or frozen for up to 3 weeks.