Preheat oven to 375°F. Grease an 8-inch round baking pan and line the bottom with parchment paper.
In a medium bowl, combine the chopped chocolate and butter. Microwave in 30 second intervals, stirring between each, until melted and smooth. Let cool slightly.
In a large bowl, beat the egg yolks and granulated sugar together until light and fluffy. Stir in the chocolate mixture until well combined.
In a separate bowl, beat the egg whites with a hand mixer until stiff peaks form. Gently fold the egg whites into the chocolate mixture in two additions.
Sift the cocoa powder and salt over the batter and gently fold in until just combined. Pour batter into prepared pan and bake for 25-30 minutes, until the center is almost set. The cake will be slightly puffed and cracked on top.
Allow cake to cool in the pan for 10 minutes, then invert onto a wire rack to cool completely.
Dust with powdered sugar and serve with fresh berries if desired. Cake can be refrigerated for up to 4 days.
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