In mixing bowl beat egg whites until soft peaks form (tips curl over when beaters are lifted). In small saucepan combine water and sugar; cook and stir over medium heat until sugar is dissolved and mixture comes to a boil. Clip a candy thermometer to side of pan. Cook at a steady boil over medium heat until mixture reaches 240 degrees F (about 5 minutes).
Beat egg whites on high speed, slowly pouring in the hot sugar mixture (pour against side of bowl, not the beaters). When all of sugar mixture is added, continue beating 5 minutes more to cool mixture.
In a large bowl whisk together whipping cream and mascarpone; whisk in the liqueur and 1/3 cup of the hazelnuts. Whisk in cooled egg whites. Sprinkle with remaining hazelnuts. Serve with Pumpkin Pie. Makes 10 servings.