I was up at Silverwood Scout Camp and was helping out with the girl Guides when they cooked some cakes in one of these. I had never seen it done before, but apparently the way you lay the foil on is very important! As it says, lay the foil shiney side down. This actually makes a big difference, if you put the foil on th wrong way, the box burns!
They did it slightly differently but the principle is the same.