Oil a metal 8-inch-square or 8-inch-wide round cake pan, or a 9-inch-wide, 1 1/2- to 2-inch-deep heart-shaped pan. Line pan bottom with waxed paper cut to fit; oil paper. Dust pan with cocoa, shaking out excess.
In a bowl, mix 2/3 cup cocoa, flour, soda, and salt.
In another bowl with a mixer, beat to blend sugar, 1/4 cup salad oil, egg whites, vanilla, and 1 cup water. Add flour mixture; stir to mix, then beat until batter is smooth. Scrape batter into prepared pan.
Bake in a 350° oven until cake begins to pull from pan side and springs back when lightly pressed in center, 35 to 40 minutes.
Let cool in pan on a rack for 10 minutes. Run a thin knife between cake and pan rim, invert cake onto a rack, lift off pan, and gently pull off and discard waxed paper. Let cool about 1 hour. Invert a flat plate onto cake; holding rack and plate together, turn cake over onto plate. Spread cake with whipped frosting, swirling with small spatula. Lightly dust with more cocoa.