Line a 15cm tin with greaseproof paper. Make sure your tin is the correct size. If its too small the puff candy won't set properly in the middle.(My tin was the wrong size...thats why its so puffy)
Get a large, heavy-bottomed pan (like a soup pot) and put in the syrup, sugar, butter and water and set over a medium heat. Stir until the sugar has dissolved. Then turn up the heat and bring the mixture to the boil.
Leave it to cook (without stirring) until a teaspoon of the mixture turns into a hard ball when dropped into a cup of cold water. (I think this gets called the hard crack stage). Remove the pan from the heat.
Add the vinegar and the bicarbonate of soda to the mixture. Be very careful when doing this as the mixture will rise VERY fast. Pour it immediately into the lined tin and leave it to the side to set.
Once the mixture has begun to set you can score it into squares to make it easier to break up later on. Leave to completely set. This is brilliant dipped in melted chocolate.