1. Butter 8-inch square pan.
2. Cook syrup to boiling in 3-quart saucepan over medium heat or place in large microwave-safe bowl and microwave at HIGH (100%) 1 minute or to boiling. Remove from heat or microwave oven; add chocolate pieces and vanilla, stirring until chocolate is melted. Gradually fold in cereal until coated with chocolate. Fold in marshmallows.
3. Press mixture into prepared pan; refrigerate until firm, about 30 minutes. Cut into squares. Cover; refrigerate leftover squares to retain crispness. Makes about 24 squares.