Melt the butter, chocolate and syrup in a heavy based saucepan. Scoop out about 1/2 cup of this melted mixture and put aside.
Put the biscuits/cookies into a freezer bag and then bash them with a rolling pin. You are looking for both crumbs and pieces of cookies.
Fold the cookie pieces and crumbs into the melted chocolate mixture, and then add the marshmallows.
Pour into a 9-inch square foil tray; flatten as best you can with a spatula. Pour over the reserved 1/2 half cup of melted chocolate mixture and smooth the top.
Refrigerate for about 2 hours or overnight.
Cut into 24 fingers and dust with powdered sugar by pushing it gently through a tea strainer or small sieve.