In a heavy-bottomed saucepan over medium heat, heat the sugar. Stir constantly with a heat-resistant rubber spatula.
The sugar will clump, but it will eventually melt. Keep stirring until the sugar melts into a light brown liquid, about 3-4 minutes.
Once the sugar melts, immediately add in butter and vanilla bean seeds (if using). Stir constantly until butter is melted, about 1-2 minutes.
Slowly pour in heavy cream while stirring constantly. Continue stirring until mixture comes up to a boil. Keep stirring and boil for 1 minute.
Remove pan from heat and add salt, stirring until combined.
Let sauce cool for 10 minutes, then pour into a glass jar. Cool completely with the lid open, then cover and store in the refrigerator. The salted caramel sauce will thicken as it cools.
With this salted caramel recipe, you need to stir constantly the entire time it's on the stovetop. It takes only 7 minutes, so put down your phone and make sure the kids are occupied for the next 7 minutes. You want to keep this mixture moving because it can burn very quickly.
If you use unsalted butter, add 3/4 teaspoon of salt to the caramel sauce in step 5. If you use salted butter, add 1/2 teaspoon salt to the caramel sauce in step 5.
The vanilla bean seeds are completely optional in this recipe. I like using them because I feel they heighten the luscious caramel flavor! You can use 1/2 teaspoon of vanilla bean paste instead of the vanilla bean seeds.
I do not recommend doubling this recipe. I've tried to a few times, and the sugar doesn't melt properly. I end up with shards of sugar in my sauce, which is not ideal at all!
When I want to make a lot of this salted caramel sauce for gifting or whatnot, I just make back-to-back batches.
You can keep this salted caramel sauce at room temperature for about a day—anything longer than that, and you'll want to keep it in the refrigerator.
This sauce will keep, covered in an air-tight jar, for about 2-3 weeks, but I doubt it will last that long!