Spray an 8-inch baking pan with non-stick cooking spray. Line the pan with parchment paper with two ends extending over 2 edges of the pan. Spray the parchment on the bottom of the pan with non stick cooking spray then use another piece of parchment to line the bottom and the other 2 sides of the pan. You should now have 4 parchment paper extensions hanging over the edges of your pan. (The bottom will be double-lined.) Set aside.
Combine all ingredients in a large microwave-safe bowl (at least 2 quarts) except the vanilla extract and the flaky salt. Stir to combine (don’t worry about the butter at this point - it well melt as the mixture cooks. Place in the microwave and cook on high power for 5 minutes then stir well to combine.
Continue cooking for 5-minute intervals, stirring in between. After 15 minutes, begin checking the mixture with an instant thermometer after each stirring.
When the temperature rises to 235˚F (112˚C), check the mixture every 2-3 minutes as the temperature can rise quickly at this point. Your goal is 243-245˚F (117-118.3˚F).
In my 900-watt microwave, it takes right around 27 minutes in total to reach the correct temperature. Your time may be longer or shorter. That's why it's important to use an instant thermometer to check the temp as you go.
Carefully remove the bowl from the microwave with oven mitts. (The mixture will be extremely hot!) Add the vanilla and stir. The caramel will bubble furiously when you add the vanilla, that’s normal! Stir until the bubbling settles down and the caramel gets nice and smooth.
Transfer the caramel into the prepared baking dish. Cool at room temperature for 30 minutes the refrigerate until completely cool, at least 1 hour. This will make the cutting much easier.
Cool to room temperature, wrap, and chill in the refrigerator until cold, about 30 minutes. This will make the cutting easier.
Sprinkle with flaky sea salt, if desired.
Spray a long, sharp knife with cooking spray. Wipe off any excess with a paper towel. then cut into squares or rectangles. I like to cut my caramels into 1=inch squares. If the knife blade gets sticky, clean it with a hot damp cloth and spray again with cooking spray.
Wrap the caramels, if desired (see the post for how to wrap) or place on a parchment lined serving tray. Enjoy!