Line a rimmed baking sheet with parchment paper and set it near the stove.
Combine the granulated sugar, unsalted butter, golden syrup, water, and fine salt in a medium heavy-bottomed saucepan.
Set over medium heat and cook, stirring gently with a heatproof spatula, until the butter melts and the mixture is homogeneous, 3–5 minutes.
Clip a candy thermometer to the pan. Increase heat to medium-high and bring to a steady boil. Do not stir once boiling; swirl the pan occasionally and brush down any sugar crystals on the sides with a wet pastry brush.
Continue boiling until the syrup reaches 295–305°F (hard-crack stage), 10–15 minutes, and the color is deep amber with a nutty aroma.
Immediately pour the hot toffee onto the prepared baking sheet and tilt the pan to spread it in an even layer about 0.25 inch thick. Do not scrape the pan’s bottom.
Cool undisturbed until fully firm, 30–45 minutes, then break into pieces.
Store airtight at cool room temperature for up to 2 weeks.