In a medium-large sauce pan (about 2-3 quart), stir the sugar over medium-high heat until melted. (You may get some sugar chunks, just keep stirring)
Once sugar is melted, continue to stir slowly and watch very closely - as soon as the sugar turns a golden/amber in color add the butter and whisk vigorously. Warning: It will bubble up quite a bit and this is normal, just keep whisking until all the butter has melted!
Remove pan from heat and slowly pour in the cream while whisking vigorously. Mixture will bubble aggressively again but continue to whisk anyway until all the cream is incorporated, then immediately whisk in the vanilla extract and the sea salt (if making salted caramel sauce).
Set the sauce aside and allow to cool for 15 minutes, then pour into a glass jar to cool completely.
Store the sauce in an airtight container in the fridge (I keep mine in a jar with a lid, it's perfect) for up to 2-3 weeks. To reuse, simply heat the sauce pack up in a sauce pan over low heat or in the microwave at 5-10 second intervals, stirring throughout. Enjoy!