Adjust oven rack to lower-middle position and heat oven to 325° F
Grease the bottom of a springform pan, line with parchment paper, then grease the side. Wrap the outside of the pan with two 18" square pieces of aluminum foil, set in roasting pan.
Bring a kettle of water to a boil
Using a stand mixer fitted with a whisk attachment, whip the eggs on medium speed until doubled in volume (~ 5 minutes)
Meanwhile, combine the chocolate, butter, and coffee in a large heatproof bowl set over a saucepan filled with one inch of barely simmering water, making sure that the water doesn’t touch the bottom of the bowl. Stirring once or twice until the chocolate and butter are melted and the mix is very warm (should be about 115°)
Using a large rubber spatula, fold one-third of the egg foam into the chocolate mix until a few streaks of egg are visible. Fold in the remaining foam in 2 additions until the mix is completely homogenous
Transfer the batter to a prepared springform pan and smooth the top with a rubber spatula.
Set the roasting pan on the oven rack and pour enough boiling water into the roasting pan 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, and a thin glazed crust (like brownie crust) has formed on the surface, and cake registers 140°. (~ 22 to 25 min)
Remove springform pan from the water bath and set on wire rack; let cool completely
Wrap springform pan tightly in plastic wrap and refrigerate until thoroughly chilled, at least 4 hours or up to 4 days. (Cake can be wrapped in an additional layer of foil and frozen for up to 1 month. If frozen, let the cake thaw in the refrigerator for 24 hours before proceeding with the next step)
About 30 minutes before serving, run a thin knife between the cake and the side of the pan; remove side of the pan
Invert cake onto a sheet of parchment and discard parchment on the bottom of the cake. Re-invert the cake onto a serving platter, discarding parchment
Dust with powdered sugar, if using, and serve
(Even though the cake may not look done, pull it from the oven when it registers 140 degrees. It will continue to firm up as it cools)
If you use a 9" springform pan instead of the preferred 8" pan, reduce the baking time to 18 – 20 minutes.
Serve with whipped cream or fresh berries