Butter a 9 x 13-inch baking dish. Use approximately 1 tablespoon of softened butter to coat the bottom and sides of the dish. Apply with a piece of wax paper, parchment paper, or clean fingers to ensure an even layer and prevent sticking.
In a large saucepan melt 1 cup unsalted butter over medium heat.
Once melted, add 1 (14 ounce can) sweetened condensed milk, 1 cup light corn syrup, and 2 cups brown sugar. Stir constantly with a rubber spatula, scraping the sides and bottom of the pan regularly.
Continue to cook the mixture over medium heat, stirring constantly, until it comes to a gentle simmer.
Maintain a steady simmer and continue stirring constantly. Cook until the mixture reaches the soft-ball stage, between 235°F and 245°F (113°C and 118°C). At higher elevations, aim for the lower end of the temperature range; at sea level or lower elevations, use the upper end.
Once the caramel reaches the correct temperature, immediately remove the pan from heat. Stir in 2 teaspoons vanilla extract and 1/4 teaspoon salt until fully incorporated.
Carefully pour the hot caramel into the prepared baking dish. Let the caramel cool completely at room temperature, about 2 to 3 hours.
Once cooled, cut into 1-inch squares using a sharp knife. For individual servings, wrap each piece in wax paper.
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Notes
Serve at room temperature. Store wrapped pieces in an airtight container at room temperature for up to 2 weeks.
Don't have a candy thermometer? You can determine if your caramel is ready the old fashioned way! Fill a drinking glass or bowl with cold water. Add in just a drop of the caramel from the pan on the stove. For the soft-ball stage, the caramel should form a small round ball in the cold water, then when it is removed, it should flatten.
Serving size is based on cutting caramels into 1-inch squares. A 9x13-inch pan will produce approximately 117 1-inch squares.