Preheat the oven to 350°F (175°C). Line an 8-inch (20cm) springform pan with parchment paper. Do not grease or butter the pan.

Melt the chocolate in a double boiler, or a bowl set over (but not touching) simmering water. Once the chocolate melts, remove from the heat and stir in the butter until smooth.

Separate 4 of the eggs. In a large bowl, whisk 2 whole eggs and the 4 egg yolks with 1/2 cup (100g) of the sugar just until combined. (Don't let the sugar sit in the eggs too long without whisking.) Whisk the warm chocolate mixture into the egg mixture. Whisk in the Grand Marnier and the orange zest, if using. In a separate bowl, beat the 4 egg whites until foamy, about 2 minutes. Gradually add the remaining 1/2 cup (100g) sugar and beat until beautifully glossy, soft peaks form that hold their shape but aren’t stiff, about 5 minutes more. Gently stir about a quarter of the beaten egg whites into the chocolate mixture to lighten it, then gently fold in the remaining whites. Pour the batter into the pan and smooth the top.

Let the cake cool in the pan on a rack. The center of the cake will sink as it cools, forming a crater. Let cool completely.

To make the Chantilly Cream, whip the cream, confectioners’ sugar, and vanilla in a large bowl until soft peaks form. Do not over whip.

Fill the sunken center of the cake with the Chantilly Cream. Dust the top lightly with cocoa powder and sprinkle chocolate shavings or chocolate curls on top.

Run the tip of a knife around the edge of the cake. Carefully remove the sides of the pan, cut into wedges, and serve.