Preheat oven to 350°. Coat a 10-in. springform pan with cooking spray.
In a food processor, pulse almonds and bittersweet chocolate until finely ground. Add cocoa and pulse to combine.
In a medium bowl, whisk together egg yolks, sugar, orange juice, orange zest, and lemon zest until the mixture is thick and pale. Beat in the chocolate-almond mixture and melted butter.
In a clean bowl with clean beaters, beat egg whites and salt until stiff peaks form; do not overbeat.
Gently fold whites into chocolate mixture, then pour into the prepared pan.
Bake until cake pulls away from edges of pan and top is well browned, about 50 minutes. Let cool completely on a rack. To serve, run a knife around cake, release pan, and cut cake into wedges. Garnish with a light dusting of cocoa powder if you like.
Note: Nutritional analysis is per serving.
Servings Makes 8 servings
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.