Stir in the corn syrup, brown sugar, and granulated sugar and continue microwaving on full power for 1-2 minutes (until boiling).
Whisk the sweetened condensed milk into the boiling sugar mixture.
Cook for 3 1⁄2 minutes longer, whisk, and cook for another 3-4 minutes. The temperature of the caramel should reach 240-248°F for a thick, chewy caramel.
Stir in the vanilla extract and salt, if desired.
Butter a jelly roll pan or half sheet pan (or a 9x13 casserole dish) or line with parchment paper.
Pour the cooked caramel mixture into the pan. Allow caramel to cool completely (1-2 hours).
Use a butter knife to slice the caramel into rectangular (or square) pieces and wrap in squares of parchment paper.
If the caramels are too soft, the caramel did not get hot enough. I found that the caramel needs to reach at least 240°F - the caramels are still edible and delicious when they are softer, but they don’t hold their shape really well and stick to the papers when wrapped.
Monitor the caramel while it is cooking. When you stir midway through the cook time, use a spatula and make sure to really scrape down the sides and across the bottom of the bowl to make sure the mixture is stirred evenly.
Caramels can be stored in the refrigerator or in a cool room.