Line a 9-inch (22 cm) baking tray with parchment paper and grease thoroughly.
Set aside a metal whisk and one tablespoon of baking soda.
In a large saucepan (the bigger, the better, as this will more than triple in size) over medium-high heat, combine sugar, corn syrup, and water.
Stir to incorporate, then leave to boil. Insert candy thermometer and continue to heat without stirring until the mixture reaches 300˚F (150˚C).
Remove from heat, and working quickly, add the baking soda and quickly whisk into the sugar syrup (whisking for no more than five seconds to prevent deflating the toffee). The mixture will begin to expand. Quickly pour it into the prepared pan and leave to set. Do not move it for 1-2 hours.
Once the toffee has cooled and set, remove from pan. Using a blunt object, shatter the toffee into bite-size pieces.
Melt chocolate in a microwave-safe bowl for 75-90 seconds. Make sure to stop and stir after every 30 seconds.
Dip the toffee pieces in chocolate and place on a lined baking tray. Finish with a sprinkle of sea salt and place in the refrigerator to cool for 15 minutes.