Line an 8" square baking pan with a sheet of parchment paper. Set aside, so it’s ready as soon as the caramel mixture is done.: The room will smell neutral at this point, but preparing the pan ahead is practical and prevents frantic moves when the caramel is hot and glossy. Make sure the parchment lies flat, creasing it slightly to fit the corners for an even surface, and leave an overhang on two sides to make lifting the set caramel easy. If you skip this, the caramel will adhere to the pan and be much harder to remove. A common mistake is using non-microwave safe containers later, so keep the pan prep separate and ready. Visual cue to look for are the straight edges and a smooth interior, which will yield neat slices once cooled.
Make sure the bowl you will be making the caramels in is microwave safe and clean and dry. Place butter (chopped into tablespoon-size pieces) in the bowl and microwave for 1 minute. It will be almost all melted. Add white and brown sugar, corn syrup, sweetened condensed milk and salt and stir well.: You will notice the butter melting into a golden pool with a gentle sizzle at the edges, and the aromas of butter will start to fill the kitchen. Dryness of the bowl matters because water droplets can change the way the sugars heat. Chop the butter into uniform pieces so it melts evenly during that first minute. When you add the sugars and sweetened condensed milk , the mixture will look granular at first, but stirring begins the dissolution process. A typical misstep is not stirring enough after adding ingredients, which can leave pockets of unmelted sugar and lead to grainy texture. Watch for the mixture to start to look homogeneous and glossy before moving to the next microwave stage.
Place mixture in a bowl into the microwave and cook for 2 minutes. Stir well. Set timer to cook for 4 minutes, but stop it every 1 minute and 30 seconds to stir the mixture. It will bubble up and rise up in the bowl, so stir it until it calms down.: During these cycles you will see vigorous bubbling, sometimes rising toward the rim, with steam releasing as the sugars concentrate. The sound is active and lively, almost like a gentle roar, and the color deepens toward amber. Stirring frequently prevents hot spots and scorching, and it also calms the foam so the bowl does not boil over. If you do not stir often, the mixture can overheat in one spot and catch, producing a burnt note. A useful sensory cue is the consistency: after stirring it should become thicker and slower to flow, and the aroma will change from sweet milky to toasty caramel. Be careful when reaching into the microwave, as the bowl and contents are very hot.
Once done, carefully remove the bowl with caramel from the microwave, stir in vanilla extract and pour mixture into prepared pan. Let cool completely in room temperature.: The caramel at this point will be glossy and thick, and you will feel a warm, cozy aroma when you lift the bowl. Add the vanilla extract off heat to preserve its fragrance, and stir gently until it is evenly distributed. When pouring into the parchment lined pan, do it slowly to avoid splashes and to achieve an even layer. The visual cue to know it is ready to pour is a smooth, slow ribbon when you lift the spoon; the mixture should not be runny. Common mistakes include pouring into a warm pan or trying to speed cooling in the fridge which can change the set; let it cool at room temperature for the ideal texture. If bubbles appear on the surface, a gentle tap on the counter can help them dissipate.
Once the caramel is cooled, lift up the parchment paper and set on a cutting board. Slice caramel into squares and wrap each one individually.: Cooling transforms the glossy liquid into a firm but tender sheet that yields clean edges when sliced. Use a sharp knife and wipe it between cuts for neat squares. The texture should be slightly chewy, yielding under gentle pressure with a satisfying pull. If you cut too early, the caramel will deform and stick, and if you cut too late, it can be overly hard. Wrapping each piece in wax paper keeps them from sticking together and makes them great for gifting. A frequent slip is not letting the caramel reach full room temperature before slicing, which leads to sticky, messy pieces instead of tidy squares.