Heat oven to 325°F. Butter the inside of spring-form pan. Melt chocolate and butter in a medium saucepan over medium-low heat; stirring frequently until smooth. Stir in sugar, cocoa powder and salt until smooth. Remove from heat and let cool for a few minutes.
In a large bowl, beat egg whites with an electric mixer on medium-high speed until they form stiff peaks, 2-3 minutes.
Stir egg yolks into cooled chocolate mixture, then stir in about a quarter of the whipped egg whites. Scrape chocolate mixture into the remaining whipped egg whites and gently fold until mostly combined (some streaks of egg white is okay). Pour into prepared pan and bake until the center of the cake no longer wiggles, 40-45 minutes. Let cool completely in pan, on a wire rack (the cake will deflate slightly). Unlock pan and dust top of cake with cocoa powder. Cut into slices and serve with raspberry compote. Serves 10.
In a medium saucepan over medium heat, combine raspberries, blackberries, sugar, water and lime juice. Cook until you can crush the berries with the back of a spoon, 5-10 minutes. Cook for another 5 minutes to thicken a bit. Remove from the heat and set aside.
Approximate Nutrient Analysis per serving:
330 calories, 17 g fat, 10 g saturated fat, 125 mg cholesterol, 100 mg sodium, 30 g carbohydrate, 3 g fiber, 24 g sugar, 6 g protein