In a medium, heavy-bottom saucepan set over medium-high heat, combine the sugar, water, and corn syrup; stir to combine.
1 cup granulated sugar, 1 teaspoon corn syrup, ¼ cup cold water
Cook, without stirring, until mixture turns a golden, straw-color, about 6-8 minutes.
Reduce the heat to low and cook, swirling the pan occasionally (but not stirring) for 2-5 minutes. The caramel is done when an instant-read thermometer registers 360°F and the mixture is a light amber color.
Remove the mixture from the heat. Quickly, and very carefully, whisk in the butter.
3 tablespoons unsalted butter
Again, carefully and quickly, add the cream and salt, and continue whisking until the sauce is smooth.
¾ cup heavy cream, ¼ teaspoon kosher salt
Cover the sauce loosely with a paper towel to absorb steam and to avoid forming a crust while it cools. Allow the sauce to cool completely before storing.