Butter an 8-by-4-inch loaf pan and line it with 2 sheets plastic wrap, one going the long way and one going the short way, leaving an overhang of several inches on all sides.
Melt the chocolate and butter in a large heatproof bowl set over a saucepan containing about 1 1/2 inches nearly simmering water, whisking until smooth. Whisk in the sugar and Cognac, if using, until well blended.
Remove the bowl from the heat and, using an electric mixer on low speed, add the egg yolks, 1 at a time, beating well after each addition and occasionally scraping down the sides of the bowl.
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 a rubber spatula, fold 1/3 of the egg whites into the chocolate mixture. Gently fold in the remaining egg whites in 2 batches, mixing just until well blended.
Pour the mixture into the prepared pan, using a rubber spatula to smoosh it into the corners and smooth the top. Cover with the overhanging plastic wrap and then wrap the entire pan tightly with aluminum foil. Freeze for at least 6 hours. To serve, unwrap the pan, fold back the plastic wrap, and run a table knife around the edges of the pan to loosen the chocolate mousse cake. Place a serving platter over the pan, invert the pan onto it, and tug on the edges of the plastic wrap to release the cake from the pan.
Remove the plastic wrap and smooth the edges of the chocolate mousse cake with a small, warm metal spatula if necessary. Cut into 1/2-inch-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 cake.)