1.Pre-heat oven to 350 degrees, grease a large baking sheet (18 by 12-inch) that is rimmed, and line with parchment paper. . Grease parchment paper or spray lightly with cooking spray.
In a medium bowl, whisk flour, sugar, salt, and baking powder together. Add oil, water, egg, and vanilla into flour mixture and mix well.
Whip egg whites and cream of tartar on medium speed on a stand mixer fitted with the whisk attachment for about 2 minutes,then increase speed to high and whip until soft peaks form , about 2 more minutes.
Fold 1/3 of whipped egg whites into the cake mixture. Repeat until all of the egg whites have been incorporated into the batter.
Spread batter into prepared cookie sheet and bake until golden brown, about 15 minutes. The cake will be done when it sprains back when pressed lightly in the center of the cake.
Remove cake from oven and transfer to a wire rack letting cake cool for about 5 minutes. Remove cake from pan by inverting cake onto the wire rack. Remove parchment paper and allow to cool completely.
Once cake is cooled, cut cake into three inch square pieces.