Line a 9×9-inch baking dish with parchment paper, ensuring the paper covers the bottom and sides to prevent sticking. Set aside.

In a 3-quart saucepan, combine the butter, brown sugar, corn syrup, sweetened condensed milk, and salt, stirring to mix well before heating.

Place the saucepan over medium heat. Stir constantly with a wooden spoon, scraping the bottom and sides to prevent burning until the mixture comes to a full boil.

Attach a candy thermometer to the pan with the bulb fully submerged but not touching the bottom. Continue cooking and stirring frequently until the mixture reaches 236°F (soft ball stage).

Remove the pan from heat once it reaches 236°F. Carefully stir in the vanilla extract until the caramel is smooth and bubbling stops.

Immediately pour the hot caramel into the prepared baking dish, spreading evenly. Allow to cool completely at room temperature for at least 4 hours until firm.

Once cooled and firm, lift the caramel block from the pan using the parchment paper edges.

Use a buttered knife to cut into 1-inch squares. Optionally, wrap each square in waxed paper rectangles for freshness.