Melt the sugar. In a medium saucepan, heat the sugar over medium heat, stirring constantly with a heatproof spatula or wooden spoon. The sugar will clump, but continue to stir until it melts and turns a rich, golden amber color. Be careful not to burn it, but you want a deep color for more flavor.
Add the butter: Once the sugar is fully melted and a deep amber color, remove it from the heat and add the butter. Be careful, as the mixture will bubble up. Whisk until the butter is fully melted. Note, if the butter isn't fully incorporated, that's okay.
Add the cream: Next, quickly, but carefully, pour in the heavy cream while continuing to whisk the sauce. The mixture will bubble vigorously again. Stir until the cream is fully mixed in and the sauce is smooth.
Finish with vanilla & salt. Next, whisk in the vanilla extract and salt. Then, transfer the caramel to a heat-safe container and allow to cool and thicken. Place in the refrigerator to store until use. Before use, microwave for 15-30 seconds to get the best results.