Adjust oven rack to lower-middle position and preheat oven to 325 degrees F. Grease 8-inch springform pan, line the bottom and sides with parchment, then grease the parchment. Wrap the outside of the pan with a double layer of aluminum foil and place in a 9x13-inch pan. Put a small pot of water on to boil.
Using a stand mixer with a whisk attachment, beat the eggs on medium speed until doubled in volume, about 5 minutes.
Meanwhile, place the chocolate, butter and coffee in a large, heat-proof bowl set over a saucepan of barely simmering water. Stir occasionally until completely melted and smooth and very warm (the mixture should register 115 degrees F on an instant-read thermometer). Remove from the heat and stir in the salt and vanilla extract.
Using a large rubber spatula, fold one-third of the eggs into the chocolate mixture until just a few streaks of egg are visible. Add the remaining eggs in two more additions, folding until the mixture is totally combined.
Transfer the batter to the prepared springform pan and smooth the top. Pour enough boiling water into the 9x13-inch 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 glaze (like a brownie crust) has formed on the surface, about 22 to 25 minutes. The cake should register 140 degrees on a digital thermometer.
Remove the cake from the water bath and set on a wire rack to cool completely to room temperature. Once cool, cover with plastic wrap and refrigerate at least 6 hours, or overnight (the cake can be refrigerated for up to 4 days).
About 30 minutes before serving, run a thin knife between the cake and sides of the pan; remove the sides of the pan. Place the cake on a serving platter, dust with powdered sugar and serve with a dollop of fresh whipped cream, if desired.
Chocolate: Bittersweet chocolate is also known as dark chocolate. For this particular cake, I recommend using chocolate with around a 60% cacao percentage. If you prefer your cake a little more bitter, go higher; if you prefer it a little more sweet, then you can go lower. I do not recommend using milk chocolate in this particular recipe, as it would be much too sweet.
Do NOT add sugar! You might be tempted to add sugar to the recipe to sweeten it up, but it will alter the texture of the cake; instead adjust the chocolate you use, as noted above.
Coffee: Coffee does a phenomenal job of enhancing the flavor of chocolate. You cannot taste the coffee in the finished product, so even if you dislike drinking coffee, I recommend you use it here. If you cannot drink coffee for medical, religious, or other reasons, you can substitute water.
Springform Pan: This recipe calls for an 8-inch springform pan, which is less common than its 9-inch cousin. If you don’t have one, I recommend this one.
Converting to 9-inch: If you do not have an 8-inch pan and prefer not to purchase one, you can still make this recipe in a 9-inch springform pan. Be aware that it will NOT fit inside of a 9x13-inch baking pan for the water bath and you will need to use something larger, like a roasting pan. The cake takes about the same amount of time to bake, but I would start checking at the 18-minute mark just to be safe.
Storage: This cake can be stored, covered, at room temperature for up to 2 days, or in the refrigerator for up to 5 days. If you store it in the refrigerator, let it sit at room temperature for at least 30 minutes before serving.
Make-Ahead Instructions: This cake can be made up to one day in advance and kept in the refrigerator. The baked and cooled cake freezes exceptionally well. Wrap it tightly in a double layer of plastic wrap, then place in a freezer-safe zipper bag. Freeze for up to 3 months; thaw in the refrigerator overnight. (You can also freeze individual slices using the same instructions.)