Line a 17cm square tin with baking paper. Place all the ingredients in a large, non-stick saucepan over a low heat, and stir frequently to dissolve the sugar and melt the butter.
Once the sugar has dissolved, turn up the heat to medium and bring the mixture to the boil, continuously stirring. Reduce the heat slightly and let the mixture simmer for 15 minutes, stirring continuously, making sure the mixture doesn’t catch at the edges of the pan.
Remove the mixture from the heat and continue to stir for about 10 minutes. The mixture will thicken a lot during this time. It’s a lot of elbow grease, but it will be worth it! Alternatively, place the mixture in a KitchenAid and beat it for a few minutes, until thick and starting to set.
Spread the mixture in the prepared tin with the the back of a metal spoon and leave to set at room temperature for 4-6 hours.