Servings: 10 to 12

1 pound semi- or bitter- sweet chocolate, coarsely chopped 1 pound semi- or bitter- sweet chocolate, coarsely chopped
1 cup unsalted butter, cut into 16 pieces 1 cup unsalted butter, cut into 16 pieces
2 tablespoons butter, melted for coating cake pan 2 tablespoons butter, melted for coating cake pan
8 large eggs, separated, at room temperature 8 large eggs, separated, at room temperature
¾ cup superfine sugar, plus more for coating cake pan ¾ cup superfine sugar, plus more for coating cake pan
2 tablespoons cognac, dark rum, or whiskey 2 tablespoons cognac, dark rum, or whiskey
1 teaspoon pure vanilla extract 1 teaspoon pure vanilla extract
½ teaspoon cream of tartar ½ teaspoon cream of tartar

Place an oven rack on the middle shelf of the oven. Preheat the oven to 350 degrees. Brush a 10" springform pan with melted butter and coat the bottom and sides of the pan with superfine sugar. Tap out any excess sugar.
In a double boiler over barely simmering water, melt together the chocolate and 1 cup butter. Stir until smooth. Or, combine the chocolate and butter in a microwave-safe bowl. Heat, uncovered, on medium power for 1½ to 4 minutes, until the butter melts and the chocolate becomes soft and shiny. Remove the bowl from the microwave and stir until chocolate is completely melted and thoroughly combined with the butter.
In a large bowl, using a handheld mixer set at medium speed, beat the egg yolks and salt until blended. Gradually add ¼ cup sugar and beat for 4 to 6 minutes, or until the mixture is pale yellow and forms a slowly dissolving ribbon on the surface of the batter when the beaters are lifted. Whisk in the cognac and vanilla. Gradually whisk the egg yolk mixture into the melted chocolate.
In a large clean bowl, beat the egg whites and cream of tartar with an electric mixer at low speed until foamy. Increase the mixer speed to medium high and continue beating until the whites become opaque and form soft mounds in the bowl. Continue beating, adding the remaining ½ cup sugar 1 tablespoon at a time, until soft peaks form.
Stir one third of the beaten egg whites into the chocolate batter to soften it. Using a rubber spatula, gently fold in the remaining egg whites just until blended.
Spoon the cholate batter into the prepared pan. Bake for 25 to 35 minutes, or until the cake appears puffy and firm. The cake should still be moist but not liquid in the center, so -- unlike the usual cake test -- a bamboo skewer inserted into the cake should not come out clean.
Remove the cake from the oven and cool on a wire rack for 15 minutes. It will fall slightly as it cools. Remove the sides of the pan. Slice and serve the cake slightly warm or at room temperature with ice cream.