Blend hazelnuts in blender or food processor until finely chopped. Reserve 1/4 cup chopped hazelnuts for dipping cookies; set aside.
In a large mixing bowl, beat butter and sugar until light and fluffy. Reduce mixer speed to low; beat in flour, vanilla extract, egg, and chopped hazelnuts.
Spoon dough into a decorating bag with medium rosette tube. Pipe dough into 2 1/2-inch "fingers," about an inch apart, onto ungreased baking sheets. Bake cookies at 400° for 6 to 8 minutes, until lightly browned. Carefully remove to racks to cool.
In a small heavy saucepan, melt chocolate over low heat until melted. Dip both ends of a cookie into the melted chocolate; carefully scrape off excess chocolate against rim of saucepan then dip chocolate ends into reserved chopped hazelnuts. Place cookies on waxed paper to let chocolate dry. Repeat with remaining cookies. Makes about 24 hazelnut cookies.
Store cookies in tightly covered container for up to 4 days.