1/2 cup (1 stick) unsalted butter plus more for pan
1/4 cup sifted unsweetened best quality cocoa powder plus more for pan
6 ounces semisweet chocolate coarsely chopped
5 large eggs
1/2 cup sugar
1/2 teaspoon salt
sweetened whipped cream for serving
Preheat the oven to 375. Butter the bottom and sides of an 8-inch round cake pan; line bottom with a parchment round. Butter parchment and dust all over with cocoa powder shaking out excess; set aside.
In a medium heatproof bowl set over, not in, a pan of simmering water, melt chocolate and butter; cool to room temperature.
Using an electric mixer on high, beat eggs, sugar, and salt until light and foamy, about 2 minutes.
Gently fold 1/2 of egg mixture into cooled chocolate mixture to lighten the chocolate. Then gently fold chocolate mixture into remaining egg mixture until marbled. Fold in cocoa powder. Spoon into prepared pan, smoothing the top. Bake 30 to 35 minutes or until center still feels soft (a toothpick inserted comes out moist) and top has a thin crust.
Transfer to a wire rack to cool completely. Run a metal spatula around the sides of pan and invert; remove parchment. Invert again so cake is right side up. Serve with whipped cream.