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Use oil to grease a 20x30cm baking tin that is 5cm deep. Line it with baking parchment, then grease again with a little oil.
Place the sugar,vanilla, golden syrup and water into a deep, heavy-bottomed pan. (Make sure the pan is very deep as the mixture will bubble up later.) Heat gently to dissolve the sugar, then increase the heat very slightly, until the mixture has turned into a syrup.
Watch the pan from now on, boiling it steadily at a fairly low heat, until the mixture turns amber.
Boil until it reaches the hard crack stage (the temperature on a sugar thermometer reads 149 - 154 degrees C. If you don't have thermometer thenit should be the colour of honey. This will take about 15 minutes.
Turn the heat off and get a whisk to hand. Add the bicarbonate of soda and whisk it all together. It will bubble up right to the top of the pan, so take care not to burn yourself as the caramel will be very hot.
Immediately pour the toffee into the prepared tin and leave to cool completely for at least an hour.
When cold get the toffee out of the tin using a palette knife and break it up into pieces.
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