In the sauce pan, cook the golden syrup, sugar, and salt over medium heat, stirring with a silicone spatula or wooden spoon, until the mixture begins to simmer around the edges. Dip the pastry brush in water and wash the sugar and syrup from the sides of the pan to keep a crust from forming. Cover and continue cooking for about 3 minutes. Uncover the pan and wash down the sides once more. Clean the crusty bits off the spatula/spoon before using again.
Attach the candy thermometer to the pan, but don't let it touch the bottom of the pan (important so you get an accurate temperature). Cook, uncovered (without stirring) until the mixture reaches 305° F/152° C.
While the syrup mixture cooks, combine the cream and ground vanilla beans/whole split vanilla bean (not the extract) in a small saucepan and heat until tiny bubbles form around the edges of the pan. Turn off the heat and cover the pan to keep the cream hot. If using the vanilla bean, remove it before adding the cream to the sugar mixture.
When the sugar mixture reaches 305° F/152° C, turn off the heat and stir in the butter chunks. Gradually stir in the hot cream; it will bubble up and steam dramatically, so be careful. Go slowly. I just drizzle in a little cream at a time until the temperature stabilizes. Turn the burner back on and adjust it so that the mixture boils energetically but not violently. Stir until any thickened syrup at the bottom of the pan is dissolved and the mixture is smooth.
Continue to cook, stirring occasionally, until the temperature reaches 245° F/118° C. Then cook, stirring constantly, to 260° F/128° C for soft, chewy caramels or 265°F/130° C; for firmer chewy caramels. The soft caramels will be easier to cut, the firm caramels will be closer to a toffee consistency.
Remove the pan from the heat and stir in the vanilla extract (only if you did not use the ground vanilla/vanilla bean). Pour the caramel into the lined pan. Let set for 4 to 5 hours, or overnight until firm.
Lift the pan liner from the pan and invert the sheet of caramel onto a sheet of parchment paper placed on a cutting board. Peel off the foil. Cut the caramels into 1-inch squares with an oiled knife. (I poured a little canola oil into a bowl, dipped a paper towel in it, then rubbed it on the knife.) Wrap each caramel individually in wax paper or cellophane.
Fleur de Sel Caramels: Use extra salt, in the form of fleur de sel or another coarse flaked salt, to bring brings out the flavor of the caramel. Add an extra scant ¼ teaspoon of coarse sea salt to the recipe. Or, to keep the salt crunchy, let the caramel cool and firm. Then sprinkle with two pinches of flaky salt and press it in. Invert, remove the pan liner, sprinkle with more salt. Then cut and wrap the caramels in wax paper or cellophane.
Nutmeg and Vanilla Bean Caramels: Add ½ teaspoon freshly grated nutmeg to the cream before you heat it.
Cardamom Caramels: Omit the vanilla. Add ½ teaspoon slightly crushed cardamom seeds (from about 15 cardamom pods) to the cream before heating it. Strain the cream when you add it to the caramel; discard the seeds.
Caramel Sauce: Stop cooking any caramel recipe or variation when it reaches 225° F/105° C or, for a sauce that thickens like hot fudge over ice cream, 228°F/110° C. Pour it into a sauceboat to serve or into a heatproof jar for storage. The sauce can be stored in the refrigerator for ages and reheated gently in the microwave or a saucepan just until hot and flowing before use. You can stir in rum or brandy to taste. If the sauce is too thick or stiff to serve over ice cream, it can always be thinned with a little water or cream. Or, if you like a sauce that thickens more over ice cream, simmer it for a few minutes longer.