Line a large cookie sheet with foil, extending foil over the edges. Sprinkle 1/2 cup of the hazelnuts in an 8-inch square on the pan. Set aside.
Butter the sides of a heavy 1-quart saucepan. In the saucepan melt the 1/2 cup butter. Add brown sugar and water. Cook and stir over medium-high heat to boiling.
Clip a candy thermometer to the side of the pan. Cook and stir over medium heat to 280 degrees F or soft-crack stage (about 10 minutes). Remove saucepan from heat; remove thermometer. Immediately and carefully stir in liqueur; pour atop nuts on pan.
Sprinkle with chocolate pieces. Let stand for 2 minutes. When chocolate is soft, spread to cover. Sprinkle with remaining 1/2 cup nuts. Cool. (If necessary, chill several minutes to harden chocolate.) Holding onto foil, lift candy off of pan. Break into pieces. Store in an airtight container in the refrigerator for up to 2 weeks. Makes about 1-1/2 pounds (48 servings).