Combine the sugar, corn syrup and 1/4 cup water in a medium saucepan, and stir until all of the sugar looks wet. Cover, set the pan over medium-high heat, and cook, stirring occasionally, until the sugar has completely melted, about 3 minutes.
Continue to cook, covered but this time without stirred, until the mixture has thickened slightly, about 3 minutes. Remove the lid and continue cooking, without stirring but paying close attention, until the mixture turns the color of dark maple syrup, about 5 minutes more.
Take the pan off the heat and immediately and gradually pour in the cream, going slowly at first and then speeding up to a nice stready stream, whisking as you pour.
Put the pan over medium-high heat again. Attach a candy thermometer to the side of the pan. Let the mixture simmer away, stirring it occasionally, until it registers 230 F on the thermometer, about 3 minutes. Take the pan off the heat and add the butter and salt, stirring slowly but constantly until the butter has completely melted.
Let the caramel cool to room temperature, then use it right away or transfer it to an airtight container and refridgerate it for up to 2 weeks. Separation is totally normal; just make sure to stir the caramel well before using it.