Preheat oven to 320°F (160°C).
Beat egg whites with salt until frothy, then gradually add sugar and beat to medium peaks.
In another bowl, whisk yolks, oil, juice, and sift flour until smooth.
Gently fold whites into yolk mixture until combined.
Pour ¾ of batter into an ungreased angel food pan. Mix melted chocolate into remaining batter and swirl it on top.
Tap pan to release air bubbles, then bake 1 hr 20 min, until golden and a toothpick comes out clean.
Invert pan immediately to cool. Once cooled, loosen edges with a knife, remove, and dust with powdered sugar if desired.
