In a medium sauce pan, add the sugar. Turn the heat onto medium-high heat. Stir constantly with a spatula. The sugar may get clumpy before it starts melting.
When the sugar starts melting, switch to a whisk. Whisk constantly to avoid scorching the sugar. It will caramelize in seconds.
When the sugar becomes an amber color, add in the butter and whisk until the butter is melted.
Remove from heat and add the heavy cream, vanilla, and salt. Whisk until the heavy cream is incorporated into the sugar/butter mixture. It will be a thick, caramelized sauce.
Let the sauce cool for a few minutes before pouring it into a jar. Keep in the refrigerator for up to 2 weeks.