Step 1

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.

Step 2

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.

Step 3

Place sugar in bowl of a food processor; process until finely ground. Reserve 1 tablespoon ground sugar.

Step 4

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.

Step 5

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.