Heat oven to 350° F. Butter a 9-inch springform pan and dust with cocoa powder.
In a medium saucepan, heat the butter with ¼ 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 confectioners’ sugar and serve with the whipped cream.