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1
Preheat the oven to 350u00b0. Butter a 9-inch springform pan and line the bottom with a round of parchment paper. Butter the paper, then dust the pan with sugar, tapping out any excess.
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2
In a heatproof bowl set over a saucepan of simmering water, melt the chopped chocolate with the butter. Add the cocoa powder and stir until smooth. Remove from heat and allow to cool slightly.
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3
In the bowl of an electric mixer, beat the egg yolks with 2/3 cup of the sugar until pale and light, about 3 minutes. Transfer yolk mixture to a large bowl, then wash mixer bowl and beaters. 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.
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4
Fold the chocolate into the egg yolk mixture until barely combined. Fold in the egg whites just until no white streaks remain. Reserve 2 cups of the batter in a medium bowl, cover with plastic, and refrigerate. Scrape the remaining batter into the prepared pan and smooth the top with a rubber 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.
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5
Once cooled the cake will deflate and cave a bit in the center. 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.
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6
Preheat the oven to 425u00b0. Bake the cake for 10 minutes, or until a thin crust forms on top and the batter is soft and gooey beneath the crust. Let the cake cool for about 10 minutes, transfer to a serving dish then cut into wedges and serve warm with ice cream.
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7
The cake can be prepared through Step 4 and refrigerated, covered, for up to 2 days.
