Line a 9-inch round spring-form pan with parchment paper.
In a double boiler, whisking frequently, melt the chocolate with the butter. Set aside for 5 minutes to cool slightly then transfer to an electric mixer fitted with a whisk. Add the eggs and beat at the high speed for 10 minutes. Add the vanilla.
Combine the sugar and flour and gently fold this mixture into the whipped chocolate base.
Pour the batter into the prepared pan and bake until the cake is firm, but the center still moist, about 15 minutes.
Cool cake then invert onto a serving plate. Dust with confectioners' sugar and serve topped with sweetened whipped cream or in a pool of crème anglaise.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.