8 ounces unsalted butter, cut into 1/2 inch chunks
1/4 cup strong coffee or liqueur, optional
8 inch spring-form pan
Chocolate meringue, for garnish, optional
Position the oven rack to the lower third of the oven (lower center) and preheat to 325 degrees. Line bottom of spring-form pan with parchment paper and grease the sides. Set the pan on a wide sheet of heavy-duty foil and wrap the foil up the sides, repeat for double layer. Set the pan in a larger baking pan or a roasting pan, set aside.
Melt the chocolate and butter (with optional liquor or coffee) in a double boiler, over simmering water, stirring frequently until the mixture is smooth. Meanwhile, bring a kettle of water to a boil for the water-bath. Allow chocolate mixture to cool slightly.
Using a hand held mixer, whisk the eggs at high speed until the volume of the eggs doubles, about 5 minutes. Fold 1/3 of the egg foam into the chocolate mixture with a large rubber spatula until just a few streaks of egg are still visible; fold in half of the remaining foam likewise. Fold the remaining foam into the batter until completely incorporated.
Pour the batter into the prepared spring-form and smooth the surface. Pour the boiling water into the roasting pan, about halfway up the side of the spring-form pan. Set the roasting pan on the oven rack. Bake until the cake has risen slightly, edges are just beginning to set, a thin glazed crust (like a brownie) has formed on the surface, and an instant read thermometer inserted halfway into the center of the cake registers 140 degrees, about 22-25 minutes.
Remove the spring-form pan from the water bath and set on a wire rack. Cool to room temperature. Cover and refrigerate overnight to mellow. Cake can be kept covered and refrigerated up to 4 days.
About 30 minutes before serving, remove the spring-form pan sides, invert the cake on a sheet of waxed paper, peel off parchment pan liner, and turn cake right side up on a serving platter. Serve with crumbled chocolate meringue, confectioners sugar, cocoa, or crème anglaise.