1 cup (2 sticks) unsalted butter, cut into pieces, plus more for the pan
1/4 cup unsweetened cocoa powder, plus more for the pan
1 1/4 cups heavy cream
8 ounces bittersweet chocolate, chopped
5 large eggs
1 cup granulated sugar
1/2 cup crème fraîche or sour cream
1/4 cup confectioners’ sugar, plus more for dusting
Heat oven to 350° F. Butter a 9-inch springform pan and dust with cocoa powder.
In a medium saucepan, heat the butter with 1/4 cup of the heavy cream over medium-low heat until the butter is melted. Add the chocolate and stir until melted and smooth; remove from heat.
In a medium bowl, whisk together the eggs, granulated sugar, and cocoa powder. Whisk in the chocolate mixture.
Transfer the batter to the prepared pan and bake until puffed and set, 35 to 40 minutes. Let cool in the pan for 1 hour. Run a knife around the edge of the cake before unmolding.
Using an electric mixer, beat the remaining 1 cup of heavy cream with the crème fraîche and confectioners’ sugar until soft peaks form. Dust the cake with cocoa (or confectioners’ sugar) and serve with the whipped cream mixture.