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1
Preheat the oven to 350u00b0F.
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2
Line the bottom of an 8-inch springform pan with a round of wax paper; do not butter the pan.
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3
Melt the chocolate in a double boiler or in a bowl set over hot water.
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4
Remove from the heat and whisk in the butter until melted; set aside.
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5
In a bowl, whisk the two whole eggs and the 4 egg yolks with 1/2 cup of the sugar just until blended.
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6
Whisk in the warm chocolate mixture.
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7
Whisk in the optional cognac or Grand Marnier and the optional orange zest.
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8
In another bowl, with an electric mixer, beat the 4 egg whites until foamy.
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9
Gradually add the remaining 1/2 cup sugar and beat until the whites form soft mounds that hold their shape, but are not quite stiff.
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10
Stir about 1/4 of the beaten egg whites into the chocolate mixture to lighten it; gently fold in the remaining whites.
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11
Pour the batter into the pan; smooth the top.
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12
Bake until the top of the cake is puffed and cracked and the center is no longer wobbly, usually 35 to 40 minutes.
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13
Do not overbake.
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14
Cool the cake in the pan on a wire rack; the cake will sink as it cools, forming a crater with high sides.
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15
FOR THE WHIPPED CREAM TOPPING: At serving time, whip the cream with the confectioner's sugar and vanilla until not quite stiff.
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16
With a spatula, carefully fill the crater of the cake with the whipped cream, pushing it gently to the edges.
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17
Dust the top lightly with cocoa powder.
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18
Run the tip of a knife around the edges of the cake; carefully remove the sides of the pan and serve.
