Heat syrup, butter and morsels to boiling, stirring constantly. Remove from heat, stir in vanilla.

Pour over cereal; toss until coated. Fold in marshmallows, 1 cup at a time. Press in pan with buttered back of spoon. Let stand 1 hour.

Cut into 2-inch squares. Store loosely covered at room temperature up to 2 days.

Makes 24 squares.