Heat the butter, chocolate, and golden syrup in a heavy-based saucepan over a gentle heat.
Remove from heat and set aside 125ml of the melted mixture in a bowl.
Crush the biscuits in a plastic bag with a rolling pin until a mix of crumbs and pieces form.
Fold the biscuit pieces and marshmallows into the main chocolate mixture in the saucepan.
Press the mixture into a 24cm square baking tin and smooth with a wet spatula.
Spread the reserved 125ml chocolate mixture over the top and smooth evenly.
Refrigerate for two hours or overnight until firm.
Cut into 24 fingers and dust with icing sugar to serve.