Whisk flour, baking powder, and salt together.
In another bowl, whisk yolks, brown sugar, oil, and water.
Sift in dry ingredients; whisk until smooth.
In a mixer, whip egg whites + cream of tartar to soft peaks. Gradually stream in sugar; whip to stiff, glossy peaks.
Whisk ⅓ of meringue into yolk mixture to lighten, then fold in the rest gently.
Pour into an ungreased 16×10” sheet pan. Bake at 325°F ~15 min, until golden and springy.
Drop pan from 6 inches (helps prevent sinking). Cool completely before releasing.