Preheat to 350°F. Spray a 9-inch-diameter springform pan with cooking spray, then line bottom with a parchment paper round. Wrap the outside of pan tightly with 2 layers of heavy-duty foil.
Combine chocolate and butter in a medium metal bowl or double boiler. Bring a small saucepan of water to a simmer and place the bowl on top. Whisk until the mixture is melted. Set aside.
In a large bowl, whisk together eggs, brown sugar, chocolate or hazelnut liqueur. Whisk in the chocolate mixture until smooth, then stir in ground hazelnuts and salt.
Pour the batter into the prepared springform pan. Fill a roasting pan halfway with boiling water and carefully place the pan in the center (so that water reaches about halfway up the sides of the springform pan). Tent the top of the springform pan loosely with foil and bake about 90 minutes. The top will still look shiny but should be dry to the touch.
Remove the springform pan from the roasting pan and cool. Refrigerate about 3 hours before serving, then enjoy with fresh whipped cream!