How to Make It
Preheat the oven to 350°. Butter a 9-inch springform pan and line the bottom with wax paper. Butter the paper, then dust the pan with flour, tapping out any excess.
In a large saucepan, melt the chopped chocolate with the butter over moderately low heat. Add the cocoa powder and stir until smooth.
In a medium bowl, using an electric mixer, beat the egg yolks with 2/3 cup of the sugar until pale and light, about 3 minutes. In a large bowl, using clean beaters, beat the egg whites until soft peaks form. Gradually add the remaining 2/3 cup of sugar and beat until the whites are firm and glossy.
Fold the chocolate into the egg yolk mixture until barely combined. Fold in the egg whites just until no white streaks remain. Spoon 2 cups of the batter into a medium bowl and refrigerate. Scrape the remaining batter into the prepared pan and smooth the top with a spatula. Bake for 45 minutes, or until the cake is puffed and a toothpick inserted in the center comes out clean. Cool the cake completely on a wire rack.
Remove the side from the springform pan and spread the reserved cake batter over the top of the cake, leaving a 1-inch border around the edge. Refrigerate the cake for at least 1 hour.
Preheat the oven to 425°. Bake the cake for 10 minutes, or until a thin crust forms on top and the batter is soft and creamy beneath the crust. Let the cake cool for 10 minutes, then cut into wedges and serve warm with the Hazelnut Ice Cream.