1) Preheat the oven to 350°F. Lightly grease two 9" round cake pans, line with parchment, and lightly grease the parchment. Set aside.
2) Whisk together the flour, baking powder, and salt in a medium bowl; set aside.
3) Combine the egg yolks and oil. Beat at low speed, using your stand mixer or a hand mixer. Slowly add the sugar as the mixture thickens. Add the water and grapefruit juice. Beat on medium speed for 3 minutes, stopping occasionally to scrape the bottom and sides of the bowl. Gently fold in the flour mixture and grapefruit zest with a wide rubber spatula.
4) In a separate bowl, free of any oil or grease, beat the egg whites until foamy. Add the cream of tartar and continue to beat on high speed until soft peaks form.
5) Gently fold the beaten whites into the batter by thirds. Combine only until traces of white remain; don't over-mix.
6) Divide the cake batter between the two pans and bake for 20 to 25 minutes, until a cake tester tests clean and the cake begins to pull away from the sides of the pan. For a 9" x 13" pan, bake 25 to 35 minutes. For cupcakes, bake 15 to 18 minutes.
7) Cool in the pan for 10 minutes before turning out onto a rack to cool completely. Frost as desired; see suggestions in "tips" at left.
Yield: two 9" round cake layers; one 9" x 13" x 2" sheet cake, or 24 cupcakes.