In a double boiler or saucepan over a low heat, gently melt together the plain chocolate, golden syrup and unsalted butter, stirring until smooth. Remove from the heat and set aside.
Break the cookies into pieces and fold into the mixture with mini marshmallows. Tip the mixture into a 20x20cm tin lined with baking parchment. Chill for about 2 hours, until set, then chop into large chunks to serve.
200g of biscuits or fruit can also be added to create more flavour and texture.