Weigh out your bicarb of soda and keep it separate. Make sure your tray is greased before you start to cook. Use a fairly large pan as when the mixture swells up, you could be surprised just how far it comes up the side of your pan.

Put butter, caster sugar and syrup into a very thick bottomed pan and heat it gently until the sugar dissolves completely. Turn up the heat at this point and boil it without stirring for a quick boil so that the mix goes darker while it simmers and just for a few minutes. Don't let the mixture go too dark or it will begin to burn. Don't take your eyes off the pan, or let children come near it as it is boiling sugar and sugar hurts if it comes into contact with any living thing.

Now for the science bit. Take it off the heat, put the bicarbonate of soda in and quickly but calmly stir the mixture. Don't be too vigorous or you might end up with a few sugar burns, and those hurt, believe me. The mix will swell up as the chemical reaction takes place. At this point, you'll be glad you didn't use a small pan. Keep children at a safe distance.

Pour the mixture into your pre-greased tin and leave it to set for a few hours. Depending on the size of tin you use, your cinder toffee could be ready to eat in minutes. If the layer in your tin is thick, it will take a fair while to cool.

Once it's cool, melt your chocolate in a microwave or double pan method (or just put a heat proof plate on the top of simmering water in a pan, and put the chocolate in the plate). Dip your cut up pieces of cinder toffee into the chocolate and leave to cool.