Fallen Chocolate Souffle Cake

10 1/2 ounces best-quality semisweet (58%) chocolate
7 tablespoons unsalted butter
8 large eggs, separated
1/2 cup sugar divided into 2 equal parts
1 teaspoon freshly grated orange zest
1 tablespoon Grand Marnier
Confectioner's sugar, for sprinkling

Before you start:

Position a rack in the center of your oven. Preheat the oven to 350°F. Grease a 10-inch springform pan with nonstick vegetable cooking spray or butter. Line the bottom with a parchment paper round.

In the top of a double boiler set over simmering, not boiling, water, melt the chocolate with the butter, stirring until smooth.
In the bowl of an electric mixer fitted with the whip attachment, whip the egg yolks with half of the sugar until doubled in volume. Add the melted chocolate and mix until combined. Transfer the chocolate batter to a large bowl. (Wash the mixer bowl and whip attachment very well, and dry thoroughly.)
In the clean bowl of the electric mixer, fitted with the clean whip attachment, make a meringue by whipping the egg whites until foamy. In a slow steady stream, add the remaining sugar. Whip until the whites hold stiff but not dry peaks.
Briskly fold one-third of the meringue into the chocolate mixture to lighten the batter. Gently fold the remaining two-thirds of the meringue into the batter until it is just incorporated.
Pour the batter into the prepared springform pan. Spin the pan to level the batter. Bake for 65 minutes, or until a wooden skewer inserted into the center comes out clean [ours was done at 55 minutes]. The cake will fall as it cools.
When the cake is cool, release the springform ring and remove it. To release the bottom, invert the cake onto a flat plate and remove the bottom and the parchment round. Turn right side up onto a serving plate and dust with confectioner's sugar. [I did not do this and instead slid the cake, with the parchment, onto a plate. I felt the delicate crust would crack if inverted.]
Serve with vanilla ice cream or warm hot fudge sauce and garnish with fresh berries, if you'd like to make the dessert a little fancy. The cake keeps tightly wrapped in plastic wrap at room temperature for up to 3 days. For longer storage, refrigerate wrapped in plastic wrap for up to 1 week. (Leave the cake in the pan for storing; it is delicate.)