Preheat oven to 350 degrees F. Grease and line two 9x5-inch loaf pans with parchment paper.* Set aside.
In a large bowl, sift together cake flour, baking powder, salt, and 1/2 cup of sugar. Whisk in sugar. In another bowl, whisk together oil, water, vanilla, and egg yolks. Add wet yolk mixture to dry flour mixture. Mix until thick and smooth. Set aside.
In another bowl, whip egg whites on low speed until foamy. Add cream of tartar and continue to whip on low speed. Add sugar one tablespoon at a time while mixing. Once all the sugar has been added, increase mixing speed and whip to stiff peaks.
Add whipped egg whites to cake batter in three additions. Using a sturdy spatula, gently fold in 1/3 of egg whites into cake batter. Adding egg whites in three additions will ensure thorough mixing and this will also keep the volume of whipped white eggs from deflating.
Pour cake batter into prepared loaf pans. Bake 9x5-inch loaf pans for 33-35 minutes until toothpick inserted in pan comes out clean. Alternatively, bake mini 3.25x5.5-inch loaf pans for 20-25 minutes. Allow cakes to cool in pan for 3-5 minutes before unmolding.
After unmolding, liberally brush warm cakes with melted salted butter. Roll warm buttered cakes in granulated sugar. Use a serrated knife to slice cake into portions. Serve warm or at room temperature. Store any leftover cakes in an airtight container for up to 4 days.