Prepare the Pan
Line a 9×13-inch baking dish with parchment paper, allowing a little overhang so the caramels can be lifted out easily later. Fill a small glass with ice water and set nearby for testing the caramel.

Cook the Caramel Base
In a large stainless steel saucepan, whisk together the sugar, Karo syrup, and kosher salt over medium-high heat. Continue stirring until the mixture begins to bubble and the sugar has fully dissolved into a smooth, glossy syrup.

Add Cream and Butter
In a slow, steady stream, whisk in the room-temperature heavy cream. Keep the caramel gently boiling as you add it, stirring constantly so the mixture stays smooth. Add the cubed butter and continue whisking until fully melted.

Bring to Soft Ball Stage
Allow the caramel to boil, whisking frequently, for about 15 to 20 minutes. Use a candy thermometer and watch for the temperature to reach between 240°F and 245°F. If you don’t have a thermometer, drizzle a small spoonful into the ice water. When it forms a firm yet pliable ball that feels slightly sticky, it’s ready.

Finish and Set
Remove from heat immediately and stir in the vanilla extract. Carefully pour the caramel into the prepared pan, smoothing gently if needed. Let the caramel cool completely at room temperature. I usually make mine in the evening and allow it to set overnight so it slices cleanly the next day.

Cut and Wrap
Once fully set, lift the caramel slab from the pan and cut into even rows using a buttered knife. Slice each row into small squares. Wrap individually in wax paper or parchment. Stored at room temperature in an airtight container, these soft caramels stay tender for up to two to three weeks.