1 pound bittersweet or semisweet chocolate, chopped
1 cup (2 sticks) unsalted butter, at room temperature
1/4 cup granulated sugar
1 tablespoon cognac or other brandy or liqueur (optional)
6 large eggs, separated
2 large egg yolks
Pinch of salt
1. Butter an 8"-by-4" loaf pan and line with 2 sheets of plastic wrap, one going the long way and one going the short way, leaving an overhang of several inches on all sides.
2. Melt the chocolate and butter in a large heat proof bowl set over a saucepan of about 11/2" of nearly simmering water, whisking until smooth. Whisk in the sugar and the cognac, if using, until well blended.
3. Remove the bowl from the heat and with an electric mixer on low speed, beat in egg yolks one at a time, beating well after each addition and occasionally scraping down the sides of the bowl.
4. Beat the egg whites and salt with clean beaters on medium-high speed in a large deep bowl just until the whites form stiff peaks when the beaters are lifted. With a whisk or rubber spatula, fold one-third of the egg whites into the chocolate mixture. Fold in the remaining egg whites in 2 batches just until well blended.
5. Transfer the mixture to the prepared pan, pushing it into the corners and smoothing the top with a rubber spatula. Cover with the overhanging plastic wrap and then wrap tightly with aluminum foil. Freeze for at least 6 hours.
6. To serve, unwrap the pan, fold back the plastic wrap, and run a table knife around the edges of the pan to loosen the marquise. Place a serving platter over the pan, invert the pan onto it, and release the marquise by pulling on the edges of the plastic wrap and smooth the edges of the marquise with a small warm metal spatula if necessary. Cut into 1/2"-thick slices with a sharp knife dipped in very warm water and wiped dry between each slice, and place on chilled dessert plates. (You can tightly rewrap and freeze any leftover marquise.)
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