Preheat the oven to 175°C. 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 (spray the bottom with nonstick spray) 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 unmold.
While the cake is cooking, make the whipped cream. Whip the cream until it becomes light and fluffy. Dust the cake with icing sugar.
Serve at room temperature with the whipped cream.
*Cook's Note: Food Network Kitchens tested this recipe after this show was taped and determined that 1 stick butter is the correct amount.