Special equipment: A 9-inch spring-form pan
Preheat the oven to 325 degrees F and butter the inside of a spring-form pan. Melt the chocolate and butter in a medium saucepan over medium-low heat; stirring frequently until smooth. Stir in the sugar, cocoa powder and salt until all of the lumps are gone. Remove from the heat and let cool for a few minutes.
Meanwhile, beat the egg whites in a large bowl with an electric mixer on medium-high speed until they form stiff peaks, 2 to 3 minutes.
Stir the egg yolks into the cooled chocolate mixture, then stir in about a quarter of the whipped egg whites. Scrape this lightened chocolate mixture into the remaining whipped egg whites and gently fold until mostly combined (it's ok if there are some streaks of egg white). Pour into the prepared pan and bake until the center of the cake no longer wiggles, 40 to 45 minutes. Let cool completely in the pan, on a wire rack (the cake will deflate slightly). Unlock the pan and dust the top of the cake with some cocoa powder. Cut into slices and serve with whipped cream.
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