Combine the lemon juice, butter, and vanilla; brush over pear halves. Sprinkle with sugar. Place pears on an oiled grilling tray or directly on the grill rack. Grill 10 to 15 minutes or until pears are very lightly golden and tender, turning occasionally. Remove from grill and place on eight serving plates.
Meanwhile, heat chocolate-hazelnut spread in a small saucepan just until warm. Drizzle liqueur over warm pears, if desired. Drizzle spread over pears and sprinkle with nuts. Makes 8 servings.