Preparation

• Position a rack in the center of the oven and heat the oven to 325°F. Butter three 8- or 9-inch round cake pans, line the bottoms with parchment paper, and butter the parchment. Dust with flour and knock out the excess flour.
• Sift the flours, baking powder, baking soda, and salt into a medium bowl. Set aside.
• In a small bowl, whisk together the water, milk, and buttermilk.
• In a stand mixer fitted with the paddle attachment, beat the butter and shortening on medium speed until creamy, 3 to 4 minutes. Add the sugar and vanilla, and beat on medium speed until fluffy, about 3 minutes. Scrape down the bowl, add the egg, and beat until just combined.
• On low speed, add the flour mixture in three additions, alternating with the water mixture in two additions, beginning and ending with the flour. Increase the speed to medium for a few seconds to incorporate. Scrape down the bowl, then mix on low speed for a few more seconds.
• In a medium bowl with a hand mixer, beat the egg whites and cream of tartar on medium speed until soft peaks form; do not overbeat. With a silicone spatula, gently fold the egg whites into the batter.
• Divide the batter among the prepared pans, and smooth with a small offset spatula. Bake, rotating the pans halfway through the baking time, until a toothpick inserted in the center of each cake comes out clean, 35 to 45 minutes. Transfer the cakes to a wire rack and let cool for 20 minutes. Invert the cakes onto the rack, remove from the pans, and let cool completely. Remove the parchment.
• Frost withVery Vanilla Frostingor with another favorite frosting.

Make Ahead Tips

的cake layers may be wrapped tightly and frozen for up to 1 month. Let thaw for a couple of hours before frosting; a partially frozen cake is easier to frost because the crumbs don’t come off as easily.