1. Adjust oven rack to lower-middle position and heat oven to 325°F. Grease 1 light-colored 9-inch round cake pan, line with parchment paper, grease parchment, and flour pan. Reserve 3 tablespoons sugar. Whisk flour, baking powder, salt, and remaining sugar together in large bowl. Add 2 whole eggs and 3 yolks (reserving whites), melted butter, water, and vanilla and whisk until smooth.

2. Using stand mixer fitted with whisk, whip remaining 3 egg whites on medium-low speed until foamy, about 1 minute. Increase speed to medium-high and whip whites to soft, billowy mounds, about 1 minute. Gradually add reserved 3 tablespoons sugar and whip until glossy, soft peaks form, 1 to 2 minutes. Stir one-third of whites into batter to lighten; gently fold remaining whites into batter until no white streaks remain.

3. Pour batter into prepared pan. Bake until toothpick inserted in center comes out clean, 30 to 40 minutes, rotating pan halfway through baking. Let cake cool in pan on wire rack for 10 minutes. Remove cake from pan, discarding parchment, and let cool completely on rack, about 2 hours.

Makes one 9-inch cake layer