Preheat oven to 325°F. Coat bottom and sides of a 9-inch spring-form pan with cooking spray. Sprinkle with cocoa; tap out excess.
Place chocolate in a heatproof bowl set over a pan of simmering water. Cook, stirring occasionally, until melted and smooth. Remove bowl from top of pan; let chocolate cool to room temperature.
Place sugar in bowl of a food processor; process until finely ground. Reserve 1 tablespoon ground sugar.
Beat butter and remaining ground sugar with an electric mixer on medium speed until light and fluffy, about 4 minutes. Add yolks, 1 at a time, beating well after each addition. Beat in vanilla. Using a rubber spatula, fold melted chocolate into yolk mixture. Fold in flour, maca powder, and salt.
Using clean, dry beaters, beat egg whites and cream of tartar on medium speed until foamy, about 1 minute. Increase speed to high and beat until medium peaks form. Stir 1/4 egg white mixture into chocolate mixture. Gently fold in remaining egg white mixture. Scrape batter into prepared pan; smooth top. Bake until a wooden pick inserted in the center comes out with moist crumbs, 35 to 40 minutes. Let cake cool in pan on a wire rack for 15 minutes. Run a knife around edge of pan to loosen cake; remove sides of pan. Let cake cool completely on wire rack, about 30 minutes. Sprinkle reserved 1 tablespoon ground sugar over top of cake; serve at room temperature.
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