Preheat your oven to 350°F. Line a 13″ x 18″ (half sheet) baking tray with parchment paper.
In one bowl, whisk together flour, cocoa powder, and salt, making sure there are no lumps.
In a larger bowl, whip egg whites to stiff peaks with a hand mixer. Slowly add in powdered sugar and egg yolk until combined.
Gently fold dry mixture into whipped egg with a spatula.
Pour mixture onto baking tray and bake for 8–10 minutes or until a fork comes out clean. (Don’t overdo it here. The key is a thin, flexible sponge so that it rolls well.)
Dust a new piece of parchment paper with powdered sugar. Flip the hot cake onto dusted parchment. Carefully roll cake on the long side, keeping the parchment paper inside the roll. Set aside to cool.