Preheat the oven to 350ºF. Prepare a 9-inch springform pan by lining with parchment circle on the bottom, and spray the inside with nonstick cooking spray. Wrap the outside of the pan tightly with 2 pieces of overlapping foil.
Place the toasted hazelnuts in a food processor and process them until they’re very fine crumbs; set aside.
Combine the chocolate and cubed butter in a medium microwave-safe bowl. Microwave in 30-second increments, stirring after every 30 seconds, until the chocolate and butter are melted together and are smooth.
In a large bowl, combine the eggs, brown sugar, and coffee. Whisk together until smooth.
Add the melted chocolate to the eggs and whisk until smooth and shiny. Finally, add the ground hazelnuts and salt, and stir until everything is well-combined.
Pour the cake batter in the pan, and put it in a large roasting pan. Pour enough hot water in the roasting pan to come halfway up the side of the springform pan.
Bake the cake for 90 minutes, until the top looks dry but still shiny, and it has a very slight jiggle when the pan is tapped. Remove the pan from the oven and take the cake out of the water bath. Once cooled to room temperature, refrigerate it for 1 to 2 hours until completely cold.
Release the sides of the pan and remove the cake. Peel the parchment from the bottom and serve. To serve, cut cake and divide slices among serving plates. Top with your choice of toppings.