Preheat oven to 350 degrees F. Line the bottom of a 9 inch springform pan with parchment paper.
Melt the chocolate in a double boiler or microwave, then let the butter melt in the warm chocolate.
Beat the two whole eggs and the four egg yolks with 1/3 cup of the sugar, then gently add the chocolate mixture, Cointreau and orange zest.
In another bowl, whisk the four egg whites until foamy, then gradually add the remaining 1/2 cp sugar and whisk until the whites are holding their shape, but not too stiff. Lighten the chocolate mixture with a dollop of egg whites, then fold in the rest of the whites.
Pour mixture into the prepare pan and bake for 35 to 40 minutes or until the cake has risen and cracked and the centre is no longer wobbly.
Cool the cake in its pan on a wire rack; the middle will sink as it cools.
When ready to eat, place the cake on a cake stand and carefully remove it from its springform pan. Don't worry about cracks or rough edges; it's the crater look that you want to go for.
Whip the cream until soft, then add vanilla and Cointreau and continue whisking until the cream is firm but not stiff.
Fill the crater of the cake with the whipped cream, easing it out gently towards the edges of the cake, and dust the top lightly with cocoa powder pushed through a strainer.