1. Mix both sugars and 1/2 cup water in a heavy large saucepan. Place over medium-high heat and stir until sugar has dissolved. Brush down sides of pan with a wet pastry brush and cook, swirling pan occasionally but not stirring, until caramel is amber in color, 8 to 10 minutes. Watch caramel carefully, as it can burn very quickly.
2. Remove from heat; add cream and butter (mixture will splatter). Stir until butter has melted. Return to heat, bring to a boil and cook for 5 minutes, until thickened and golden blond in color. Remove from heat and stir in bourbon and salt. Serve warm or at room temperature, or transfer to a jar, let cool, cover and refrigerate for up to 2 weeks.