Melt the butter and combine with the biscuit crumbs. Press mixture into bottom of 23cm round springform tin. Bake at 180 C / Gas 4 for 10 minutes.
In a small saucepan, soften gelatine in water; stir over low heat until dissolved.
In a large bowl, combine cream cheese, sugar, cocoa and vanilla; blend with an electric mixer on medium speed. Gradually add gelatine mixture and blend. Fold in whipped cream, 100g marshmallows and walnuts; pour onto biscuit base. Chill until firm.