Butter, spray or line an 8 by 8-inch baking dish.
Place butter in a large microwave safe bowl (glass will work best) until melted through.
Add remaining ingredients (corn syrup, white and brown sugar, sweetened condensed milk) and stir until smooth.
Return bowl to microwave and cook for six to seven minutes at full power, depending on the temperament of your microwave. Be careful not to overcook your caramel mixture or the candy will be too hard. (If you'd like to test your caramel with a candy thermometer, it should read about 240 degrees.)
Using a hot pad, carefully remove the bowl from microwave. Add in vanilla (and optional nuts) and mix well.
Pour the mixture into the baking dish. Sprinkle top lightly with sea salt or chocolate drizzle.
Set aside for 20-30 minutes to cool. To speed up the process, place the caramels into the refrigerator.
Once the candy has cooled completely cut to desired shape with a plastic knife and serve immediately or wrap in waxed paper or plastic wrap.
Caramels will keep for approximately one to two weeks.