1.Grease two 8cm x 25cm (3½-inch x 10-inch) bar pans; line bases with baking paper, extending paper 5cm (2 inches) over long sides.
2.Stir marshmallows, the water and butter in medium saucepan over low heat until marshmallows are melted. Remove from heat; stir in chocolate until smooth. Stir in biscuits, cherries and nuts.
3.Spread mixture evenly between pans (do not crush biscuits). Cover; refrigerate 1 hour.
4.Combine extra chocolate and extra butter; spread mixture evenly over slices. Refrigerate 1 hour or until firm.
Store slice in an airtight container, in the fridge, for up to a week. Pecans can be used instead of walnuts.