Bring cream to a boil in a medium saucepan over medium-high heat; whisk in bittersweet chocolate, butter, and hazelnut liqueur until well combined. Chill at least 2 hours.
Place hazelnuts on a baking sheet. Bake at 350° for 10 minutes or until hazelnuts are toasted. Place warm hazelnuts in a dish towel, and rub vigorously to remove skins.
Process toasted hazelnuts in a food processor until ground. Place in a shallow dish.
Shape chocolate mixture into 1-inch balls. Melt chopped dark chocolate bar in a small saucepan over low heat. Roll each ball in 1 teaspoon of melted dark chocolate, and immediately roll in toasted, ground hazelnuts.
Cover and chill truffles until ready to serve.
Note: For testing purposes only, we used Ghirardelli Dark Chocolate for dark chocolate bar.
*1/2 cup semisweet morsels may be substituted for dark chocolate bar.