Preheat the oven to 350 degrees F. Butter a 9-inch springform pan.
Put the chocolate and butter into the top of a double boiler (or in a heatproof bowl) and heat over (but not touching) about 1 inch of simmering water until melted. Meanwhile, whisk the egg yolks with the sugar in a mixing bowl until light yellow in color. Whisk a little of the chocolate mixture into the egg yolk mixture to temper the eggs - this will keep the eggs from scrambling from the heat of the chocolate - then whisk in the rest of the chocolate mixture.
Beat the egg whites in a mixing bowl until stiff peaks form and fold into the chocolate mixture. Pour into the prepared pan and bake until the cake is set, the top starts to crack and a toothpick inserted into the cake comes out with moist crumbs clinging to it, 20 to 25 minutes. Let stand 10 minutes, then remove sides of pan.
While the cake is cooking, whip the cream until it becomes light and fluffy.
Serve at room temperature dusted with confectioners' sugar and topped with whipped cream.
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