Place the sugar in a heavy-bottomed saucepan over medium-high heat. Stir the sugar every minute or two with a wooden spoon until it melts into an amber-colored liquid and no sugar clumps remain—this can take up to 10-15 minutes, but make sure it does not burn.

Reduce the heat to medium. Carefully add the butter one cube at a time, as it will bubble violently. Whisk until the butter is fully melted and incorporated, around 2-3 minutes.

Steadily stream the heavy cream into the caramel, whisking constantly. As soon as the cream is incorporated, let the caramel boil for a full minute before removing it from the heat and stirring in the vanilla extract and salt. Pour the caramel into a separate bowl and let it cool completely before using (it will thicken as it cools).

The caramel will keep at room temperature for a few days, or in the fridge for around 3 weeks.