In 1-quart heavy saucepan, heat both chocolates and whipping cream over low heat, stirring constantly, until chocolate is melted and smooth. Remove from heat. Stir in butter, a few pieces at a time. Stir in liqueur. Place plastic wrap over surface of chocolate. Refrigerate about 2 hours, stirring once, until firm enough to hold its shape.
Meanwhile, heat oven to 350°F. Place hazelnuts in ungreased shallow pan. Bake 6 to 10 minutes, stirring occasionally, until light brown. Rub with towel to remove skins. Cool 10 minutes. Place nuts in food processor. Cover; process with on-and-off pulses 20 to 30 seconds or until finely ground. Place on sheet of waxed paper.
Scoop rounded teaspoonfuls of chocolate mixture onto nuts. Roll lightly to coat and shape into 1-inch balls (truffles do not need to be smooth; they should be a little rough). Place on plate; cover loosely. Store loosely covered in refrigerator. Let stand at room temperature 15 minutes before serving.