1. Butter a 9-inch (23cm) springform pan. Dust the inside with cocoa powder and tap out any excess. Preheat the oven to 350ºF (180ºC).
2. In a large, heatproof bowl set over a saucepan of barely simmering water, melt the chocolate and butter, stirring occasionally, until smooth. Remove from heat and whisk in 1/3 cup (65g) of sugar, then the egg yolks, all-purpose flour or almond flour, and vanilla.
3. In the bowl of stand mixer fitted with the whip attachment, or in a large bowl using a whisk, begin whipping the egg whites at medium speed. When they start to get frothy, increase the speed to high, add the salt, and whip until the egg whites start to hold their shape. Continue to whip, gradually add the remaining 1/3 cup (65g) of sugar, until the whites are firm enough to hold their shape, but not too stiff or dry.
4. Using a spatula, fold one-third of the egg whites in the chocolate mixture, then fold in the remaining egg whites, just until their are no visible streaks of egg whites. Scrape the batter into the prepared pan, smooth the top, and bake until the center of the cake feel almost set, but not quite, about 20 to 25 minutes. (If using almond meal, it may take up to 30 minutes.) A toothpick inserted into the center should come out with just a few moist crumbs attached.
5. Remove the cake from the oven and let cool. Once cool, run a knife around the outside of the cake to loosen it from the edges of the pan, then remove the outside of the springform pan.