Spray an 8x8-inch square pan with cooking spray. Line with parchment, so that the parchment extends up two sides of the pan, and spray again. Set aside.
Combine the sugars, corn syrup, and water in a medium saucepan, preferably one that is tall and skinny. Stir the mixture, being mindful not to splash the sides of the pan with sugar, and place it over medium to medium-high heat. Once the mixture boils, attach a candy thermometer to the side of the pan, and bring the mixture to 245 to 248 degrees (firm ball). While cooking, use a wet pastry brush to wash off any crystalizing sugar on the sides of your pan. Do not stir.
Meanwhile, place the heavy cream, condensed milk, and butter in a small saucepan, and warm this mixture on medium heat. Do not let it boil or simmer and continue to keep the mixture warm, while melting the sugars, etc.
Once the sugars reach firm ball, remove the pan from the heat and add the warm cream mixture, being careful as you add it, as the caramel may splatter and bubble. Gently stir the caramel with a wooden spoon or heatproof spatula, and return it to medium to medium-high heat. Continue cooking the caramel until it reaches firm ball again. Do not stir. This may take up to 20 minutes on medium heat.
Once it reaches temperature, immediately remove the caramel from the heat and add the vanilla and salt. Stir to combine. Pour the caramel into the prepared pan and set aside on a cooling rack until the caramel comes to room temp, several hours, or overnight.
Once cooled, lift the caramel out of the pan by the parchment paper, place it on a cutting board and slice the caramels into one-inch square pieces, using a chef's knife. Immediately wrap in small squares of parchment paper, twisting the ends of the paper to securely close. Caramel candies will keep up to a week on the counter. You can also refrigerate them, but bring to room temp before serving.