Adapted from treats-sf.com
large eggs, cold
lb bittersweet or semisweet chocolate chips
lb unsalted butter (2 sticks), cut in to pieces
cup espresso (optional)
Confectioner's sugar or cocoa powder for decoration
Adjust oven rack to lower middle position and heat oven to 325 degrees. Line bottom of 8-inch springform pan with parchment and grease pan sides. Cover pan underneath and along sides with sheet of heavy-duty foil and set in large roasting pan. Bring kettle of water to boil. Beat eggs with hand-held mixer at high speed until volume doubles to approximately 1 quart, about 5 minutes. Alternately, beat in bowl of electric mixer fitted with wire whip attachment at medium speed (speed 6 on a KitchenAid) to achieve same result, about 5 minutes. Meanwhile, melt chocolate and butter (adding coffee or liqueur, if using) in large heat-proof bowl set over pan of almost simmering water, until smooth and very warm (about 115 degrees on an instant-read thermometer), stirring once or twice. (For the microwave, melt chocolate and butter together at 50 percent power until smooth and warm, 4 to 6 minutes, stirring once or twice.) Fold 1/3 of egg foam into chocolate mixture using large rubber spatula until only a few streaks of egg are visible; fold in half of remaining foam, then last of remaining foam, until mixture is totally homogenous. Scrape batter into prepared springform pan and smooth surface with rubber spatula. Set roasting pan on oven rack and pour enough boiling water to come about halfway up side of springform pan. Bake until cake has risen slightly, edges are just beginning to set, a thin glazed crust (like a brownie) has formed on surface, and an instant read thermometer inserted halfway through center of cake registers 140 degrees, 22 to 25 minutes. Remove cake pan from water bath and set on wire rack; cool to room temperature. Cover and refrigerate overnight to mellow (can be covered and refrigerated for up to 4 days). About 30 minutes before serving, remove springform pan sides, invert cake on sheet of waxed paper, peel off parchment pan liner, and turn cake right side up on serving platter. Sieve light sprinkling of Confectioners’ sugar or unsweetened cocoa powder over cake to decorate, if desired.