Prepare the pan. Line a 9x9 or 8x8 baking pan with parchment paper, ensuring the edges extend slightly over the sides for easy removal. Set aside.
Combine the ingredients. In a 3-quart, heavy-bottomed saucepan, place the butter, brown sugar, corn syrup, sweetened condensed milk, Fireball whiskey, and salt.
Cook the mixture. Bring the mixture to a boil over medium heat. Allow it to cook without stirring until a candy thermometer reads 246°F (firm ball stage). This cooking stage will take about 10-15 minutes, and you’ll notice the mixture darkening as it cooks.Tip: If you’re using a standard saucepan (such as stainless steel), keep the heat lower and swirl the pan occasionally. Add vanilla. Once the mixture reaches 246°F, promptly remove it from the heat. Then, add the vanilla extract and stir until the caramel is smooth and no longer bubbling—this should take less than a minute.
Pour. Immediately pour the hot caramel into the prepared baking pan, smoothing it out.
Let it cool. Let the caramel cool completely at room temperature without disturbing it. Depending on your room temperature, this will take 2 to 4 hours, and it’s important to let it be undisturbed for a smooth surface.
Cut and wrap. Once cooled, use a buttered knife to cut the caramel into one-inch squares. For the best results, lightly mark the caramel with a ruler beforehand to keep your pieces uniform. Wrap each piece in wax paper to prevent sticking, and store at room temperature for up to two weeks.