In a microwave-safe glass measuring cup (at least 4 cups large) or medium bowl, add the granulated sugar, corn syrup and water. Mix to combine.
Microwave on high for 6-9 minutes, stirring once at the 3-minute mark, until the mixture is just starting to turn a light yellow color.
Leave the door closed on the microwave and let the mixture sit for about 5 minutes, until it darkens to golden amber color. Do not stir yet.
Then gradually stir in the room temperature heavy cream. Be careful, as the mixture will bubble up as the cream gets added.
Then, add the unsalted butter, vanilla extract and sea salt. Stir to combine.
Serve immediately or store in the refrigerator for up to 3 weeks until ready to use.
Notes
Corn syrup, while not an ingredient I reach for often, is very helpful in this microwaved caramel sauce. The liquid sugar helps prevent crystallization.
You'll be microwaving the ingredients for 6-9 minutes. I like to stir the mixture only 1 time about halfway through. This will ensure that each and every sugar crystal is incorporated, to keep the sauce from being grainy.
It won't look like caramel at first! The mixture will be a light yellow color after you microwave it.
The key to getting a homemade caramel sauce made in the microwave this gorgeous dark amber color is time! After the cooking time is finished, just let the caramel cool down in the microwave without touching it for 5 minutes.
If your kitchen is particularly cold, you may find that the caramel cools too fast and it's more difficult to mix in the cream at the end. Keep this in mind, and try to save this recipe for a warmer day.