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 with Very Vanilla Frosting or with another favorite frosting.
Make Ahead Tips
The 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.
nutrition information (per serving):
300, Fat Calories
13, Saturated Fat
6, Polyunsaturated Fat
3.5, Monounsaturated Fat
Photo: Scott Phillips