Grease a slice tin (approx 28 x 18cm) and line with baking paper, leaving paper overhanging at sides to assist with removal of the tablet later.
Add all ingredients apart from the vanilla essence in a large saucepan. Mix to combine and then put on stove over low heat.
Stir until the sugar is dissolved, turning the heat up to low-medium if necessary.
Slowly bring the mixture to the boil and then boil for 10-15 minutes, stirring from time to time and turning down the heat if necessary to maintain a fast, steady boil.
If you have a sugar thermometer, take the mixture off the heat when the temperature of the mixture reaches 120 degrees celsius (248 degrees fahrenheit). Otherwise, watch the mixture as it changes to a more golden colour around the 8-10 minute mark.
Once you've take the mixture off the heat, stir in the vanilla extract.
Now, get ready cause this is the hard part. Stir the mixture vigorously for 10-15 minutes or until it thickens so much you can hardly continue to stir it.
If the mixture becomes smooth after you stop stirring for a few seconds, keep going. You want it to hold its shape when you stop stirring.
Pour the mixture into the slice tin and spread out evenly with a spatula, smoothing the top as best you can.
Allow to set for at least 6 hours, preferably overnight.
Ease the entire slab from the tin using the baking paper once set and then cut into small squares.