Add flour, brown sugar and coconut to melted butter. Press into a lined slice tin. Bake in a moderate oven for 25-30 minutes.
Meanwhile, melt remaining butter, golden syrup and condensed milk until it just starts to change colour and thicken. Add to base and cook a further 15-20 minutes or until bubbling and golden.
When the base is cold, add the melted chocolate/oil and cut into squares when set.