Line a 18cm rectangle tin with greaseproof paper. Chop the chocolates up into chunks and crush the biscuits in a freezer bag until the pieces are the size of a 20p.
In a large saucepan melt 135g butter, 200g dark chocolate and 3 tbsp golden syrup over a gentle heat stirring constantly until there are no or almost no more lumps of chocolate visible, then remove from the heat. Leave to cool.
Take the biscuits, 100g mini marshmallows and the chocolate chunks and stir into the chocolate mixture until everything is completely covered.
Tip the mixture into the lined baking tin, and spread it out to the corners. Chill for at least 2 hrs then dust with icing sugar and cut into 12 fingers.