Line a 9x13-inch baking dish with parchment paper or generously butter and set aside so it is ready. Fill a small glass with ice water.
In a large, stainless steel pot, whisk together the sugar, Karo syrup, and salt over medium-high heat until bubbling the sugar has dissolved. In a very slow trickle, whisk in the cream, being careful not to stop boiling. Whisk in the butter.
Whisking constantly, keep the mixture at a boil 15 to 20 minutes, or until the temperature on a candy thermometer reads 240°F to 245°F (firm ball stage). To be certain the caramel is ready, drizzle 1 teaspoon into the ice water. Roll it between your fingers and if it forms a firm (yet pliable and little sticky) ball, it’s ready. If it’s still liquid, keep boiling and whisking.
Immediately remove from heat and whisk in the vanilla. Then pour into the baking dish. Don't let it overcook or the caramels will be too hard. Let sit on the counter to cool and until set. Usually, I will make in the evening and let sit overnight.
Once set, use a buttered or plastic knife to cut into 12 vertical rows, Cut 4 caramels from each row. Cut wax paper into 3x6-inch squares and then wrap the caramel inside. Caramels will stay soft for 2 to 3 weeks.
Notes
Note: Here’s an easy way to test if the caramel is ready. Place a teaspoon of caramel into a small glass of ice water. If the caramel forms a firm ball, it’s ready. If it’s still liquid, keep boiling and stirring. I like to use a candy thermometer as well to make sure it has reached 245°F.For best results: Do not double this recipe, use a candy thermometer and follow the tips in the post above.Storage: For best results, store in a covered container for up to 2 weeks.