Fit parchment across the bottom and up the sides of a 9-inch square pan, or line a half sheet pan with parchment or a silicone baking mat for thinner candy. The lining should cover every surface the hot mixture will touch.
Set a whisk and spatula beside the stove. Both need to be within reach when the syrup begins foaming.
Combine the sugar, golden syrup or honey, water, and salt in a large saucepan, using corn syrup only as the listed fallback. Leave generous headroom above the syrup for expansion.
Moisten the sugar with gentle stirring, then place the saucepan over medium-high heat and swirl frequently. The mixture should lose its grainy appearance as the sugar dissolves.
Wash down crystals on the saucepan walls with a pastry brush dipped in cold water, without stirring the syrup. The sides should look clear rather than crusted with sugar.
Attach a candy thermometer once the mixture boils and the sugar has dissolved. Keep the sensing tip in the syrup rather than against the pan bottom.
Continue heating to 300°F / 150°C, swirling for even heat and checking the syrup in several places with the thermometer. The syrup will become more concentrated, with its color depending on the sugar and syrup used.
Bring the lined pan close as the syrup approaches its target, and have the whisk, spatula, and baking soda ready. An oven mitt can help protect your whisking hand from the rising heat.
Take the saucepan off the burner at the target temperature and immediately whisk in all the baking soda, reaching around the edges. Stop once the soda disappears into an evenly foaming mixture.
Transfer the expanded candy promptly into the lined pan, using the spatula to help it out without pressing or spreading the top. The uneven, bubbly surface should remain undisturbed.
Leave the slab to harden for a few hours. It should feel firm throughout and sound hollow when gently tapped.
Lift out the firm candy, remove the parchment, and place the slab on a cutting board before breaking it with the tip of a sharp knife. Expect irregular chunks rather than tidy squares.