Bittersweet Chocolate Cloud Cake

8 ounces best-quality bittersweet or semisweet chocolate, coarsely chopped
1/2 cup (1 stick) unsalted butter, cut into pieces, softened
6 large eggs: 2 whole, 4 separated
1 cup sugar
2 Tablespoons cognac or Grand Marnier (optional)
Grated zest of 1 orange (optional)

Whipped Cream Topping:
1-1/2 cups heavy cream, well chilled
3 Tablespoons confectioners' sugar
1 teaspoon pure vanilla extract
Unsweetened cocoa powder, for sprinkling

Preheat the oven to 350 degrees F (175 C). Line the bottom of an 8-inch springform pan with a round of wax paper; do not butter the pan. Melt the chocolate in a bowl set over hot water. Remove from the heat and whisk in the butter until melted; set aside.

In a bowl, whisk the 2 whole eggs and the 4 egg yolks with 1/2 cup of the sugar just until blended. Whisk in the warm chocolate mixture. Whisk in the optional cognac or Grand Marnier and the optional orange zest.

In another bowl, with an electric mixer, beat the 4 egg whites until foamy. 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. Stir about 1/4 of the beaten egg whites into the chocolate mixture to lighten it; gently fold in the remaining whites. Pour the batter into the pan; smooth the top.

Bake until the top of the cake is puffed and cracked and the center is no longer wobbly, usually 35 to 40 minutes. Do not over bake.

Cool the cake in the pan on a wire rack; the cake will sink as it cools, forming a crater with high sides.

At serving time, whip the cream with the confectioners' sugar and vanilla until not quite stiff. With a spatula, carefully fill the crater of the cake with the whipped cream, pushing it gently to the edges. Dust the top lightly with cocoa powder. Run the tip of a knife around the edges of the cake; carefully remove the sides of the pan and serve.