Line a 9x9 pan with parchment paper and butter the parchment paper and set aside.
In a 2-quart or larger glass microwave-safe bowl, melt butter. (Make sure the bowl you use can hold at least 2 quarts or it will boil over.)
Next, add brown sugar, Karo syrup, and sweet and condensed milk. Stir to mix.
Microwave on high for 5 minutes. Remove and stir.
Return to microwave and microwave on high for 5 more minutes. Remove and stir.
Return to microwave again and microwave for 4 minutes. Remove and stir.
Next, return to microwave for 1 more minute. At this point, I like to do the cold water test--like I do with these caramels--to make sure they are the consistency I want them to be. If they aren't quite there yet, microwave for 1 more minute and test again. If still not quite there, microwave for 1 more minute and test again.
Stir in vanilla and pour hot caramel mixture into the prepared 9x9 pan. Sprinkle with coarse sea salt--just a little. You don't want to get more than one grain of sea salt per bite.
Put caramels in the fridge for at least 2 hours to let the caramels cool completely.
Cut cooled caramels and wrap them in wax paper.
Enjoy!
Recipe by Real Life Dinner at https://reallifedinner.com/the-best-microwave-caramels/