Heat the oven to 325°F with a rack in the middle position. Mist an 8-inch square metal baking pan or glass baking dish with cooking spray, then line it with a 7-by-18-inch sheet of kitchen parchment. In a small bowl, whisk together the flour, cocoa, and salt. Set aside.

In a stand mixer with the whisk attachment, beat the egg yolks and sugar on medium-high speed, scraping the bowl as needed, until the mixture is light, fluffy and doubled in volume, about 5 minutes.

With the mixer running on low, slowly add the butter, then mix for 1 minute. With the mixer running on steam in the milk and vanilla, mix for 1 minute. Gradually add the flour, then mix until combined, about 30 seconds. The base will be thin and dotted with beads of flour; this is fine.

Thoroughly wash and dry the mixer bowl and whisk attachment. Add the egg whites to the bowl then whisk on medium until foamy, about 1 minute. Increase to medium-high and whip until the whites hold stiff peaks, 3 to 5 minutes.

Scrape half of the whipped whites onto the base. Using a whisk, fold until the whites are broken into lumps no larger than ½ inch. Add the remaining whites and fold, still with a whisk, until the whites create curd-like lumps, no larger than ½ inch, at the surface; do not overfold. The whites will not be fully incorporated and the batter will be fluid.

Gently pour the batter into the center of the prepared pan. Bake until puffed with a slight jiggle in the center, 45 to 50 minutes. Cool to room temperature then pan on a wire rack, at least 2 hours. The cake will deflate slightly and pull away from the sides of the pan as it cools.

Run a thin-bladed knife between the cake and pan to loosen. Using the parchment overhang as handles, lift the cake out of the pan and set it on a cutting board. Slide a thin metal spatula between the cake and parchment, then slide the parchment out from underneath. Cut the cake into pieces. Dust with cocoa just before serving.