- Position oven rack to the lower-middle part of the oven and heat oven to 325°F.
- Line the bottom of an 8-inch springform pan with parchment paper or waxed paper and grease the sides of the pan.
- Beat eggs in a bowl of a standing mixer at high speed until the volume doubles, about 5 minutes.
- Meanwhile, melt the chocolate and butter (adding the coffee, if using) in a large heatproof bowl set over a pan of almost-simmering water until smooth and very warm (about 115 degrees on an instant-read thermometer), stirring once or twice. Fold a third of the egg foam into the chocolate mixture using a large rubber spatula until only a few streaks of egg are visible; fold in half of the remaining foam, then the last of the foam, until the mixture is totally incorporated.
- Scrape the batter into the prepared springform pan and smooth the surface with a rubber spatula. Set the roasting pan on the oven rack and pour in enough boiling water to come about halfway up the sides of the springform pan. Bake until the cake has risen slightly, the edges are just beginning to set, a thin glazed crust has formed on the surface, and an instant-read thermometer inserted halfway into the center reads 140°F, 22 to 25 minutes.
- Cover and refrigerate overnight.
- Dust with confectioners' sugar or unsweetened cocoa before serving, if desired.
Calories 592, Carbohydrates 45g, Cholesterol 165mg, Fat 43g, Fat Calories 385, Protein 7g, Saturated Fat 29g, Sodium 90mg. Daily Values: Calcium 36.95mg 4%, Phos 70.54mg 7%, Copper 0.03mg 2%, Zinc 0.45mg 3%, Panto 0.53mg 5%, Biotin 6.67mcg 2223%, Iodine 17.67mcg 12%, Magnesium 4.38mg 1%, Iron 1.58mg 9%, Vitamin B6 3%, Vitamin B12 6%, Vitamin D 10%, Vit E-a-Toco 3%, Vitamin A 13%. Percent Daily Values are based on a 2,000 calorie diet.