Add all of the ingredients (except for the vanilla essence) to a large saucepan. Bring to the boil and continue to boil for 20-25 minutes. Stir constantly to avoid the mixture sticking to the bottom of the saucepan. It will be ready when it reaches the soft boil stage or if you're using a candy thermometer, the temperature will reach 116°C
Pour the hot mixture into a large mixing bowl, add the vanilla essence. Beat for 5 minutes, until the mixture just starts to set
Add the mixture to a lined square baking tin and allow to completely cool and set on the bench top
Cut into squares and serve. Store in an air tight container