Notes: Make sauce up to 1 week ahead; cover and chill. It thickens as it cools. Warm in a microwave oven or stir in a pan over medium heat.
1. Put hazelnuts in an 8- or 9-inch pan. Bake in a 350° oven until nuts are golden under skins, 15 to 20 minutes. Pour onto a clean towel and, when cool enough to touch, rub nuts with fabric to loosen skins. Lift nuts from towel; discard skins. Coarsely chop nuts.
2. In a 10- to 12-inch nonstick frying pan over high heat, combine sugar and butter. Shake pan frequently to mix until sugar and butter are melted and amber colored, about 5 minutes; watch carefully.
3. Off the heat, add whipping cream (mixture foams); stir until caramel is smoothly mixed with cream. Stir in chopped nuts and liqueur.
4. Return sauce to medium heat and stir until boiling vigorously. Use sauce hot or cool.