Instructions

1. Lightly butter or line a 20cm x 20cm (8 x 8 inch) dish with baking paper and set aside. 
2. Place the butter in a large, heatproof, microwave-safe bowl and microwave until completely melted.
3. Add the corn syrup, castor sugar, brown sugar, condensed milk and salt. Whisk everything together really well until evenly combined.
4. Microwave the mixture on HIGH for approximately 6 minutes. Cooking times can vary slightly depending on the wattage of your microwave, so check the caramel at this point before cooking it any further. 
5. To test whether it’s ready, drop a small spoonful of caramel into a bowl of ice-cold water. Once cooled, it should form a soft, pliable ball — not too runny and not hard. If using a sugar thermometer, you’re looking for approximately 114–117°C (238–242°F). I have a digital point and measure thermometer which is helpful to quickly measure the temperature without taking the bowl out the microwave.
6. If it needs a little more cooking, return it to the microwave in 20-second intervals, testing again after each interval. 
7. Carefully remove the bowl from the microwave — the mixture will be extremely hot. Stir in the vanilla extract.
8. Pour the caramel into your prepared dish and smooth it out. If you like a sweet-and-salty finish, sprinkle a little Maldon salt over the top.
9. Refrigerate for 20–30 minutes, or until the caramel has set enough to slice. 
10. Lift out and cut into small rectangular pieces or squares.
11. Wrap each caramel individually in small pieces of wax paper.