1 cup sugar (7 ounces = 200 grams)
1 tablespoon Lyle's Golden Syrup (refiner's syrup) or corn syrup (0.75 ounce = 21 grams)
1/4 liquid cup water (2 ounces = 60 grams)
1/2 liquid cup heavy cream, heated (4 ounces = 116 grams)
2 tablespoons unsalted butter, softened (1 ounce = 28 grams)
1 teaspoon pure vanilla extract
EQUIPMENT A heavy saucepan, at least 5-cup capacity, ideally with a nonstick lining
In the saucepan, stir together the sugar, syrup, and water until the sugar is completely moistened. Heat, stirring constantly, until the sugar dissolves and the syrup is bubbling. Stop stirring completely and allow it to boil undisturbed until it turns a deep amber (380°F.). Immediately remove it from the heat and slowly and carefully pour the hot cream into the caramel. It will bubble up furiously.
Use a high-temperature heat-resistant rubber spatula, or a porcelain or wooden spoon to stir the mixture until smooth, scraping up the thicker part that settles on the bottom. If any lumps develop, return the pan to the heat and stir until they dissolve. Stir in the butter. The mixture will be streaky but become uniform after cooling slightly and stirring.
Allow the sauce to cool for 3 minutes. Gently stir in the vanilla extract.
Room temperature, up to 3 days; refrigerated, at least 3 months. To reheat: If the caramel is in a microwave-safe container at room temperature, microwave it on high power for 1 minute, stirring twice. If cold, it will take a few seconds more. Alternatively, place it in a bowl in a pan of simmering water and heat, stirring occasionally, until warm, about 7 minutes.