Melt the Butter: In a medium saucepan over medium-low heat, melt the butter completely. Turn off the heat.
Add Sugar and Mix: Stir in the light brown sugar and mix well until it dissolves into the butter.
Incorporate Salt and Corn Syrup: Add the salt and corn syrup, stirring continuously to combine everything smoothly.
Mix in Condensed Milk: Slowly pour in the sweetened condensed milk, stirring gently as you add it.
Bring to a Boil: Turn the heat to medium and bring the mixture to a boil. Important: Stir constantly and gently to prevent burning.
Cook to Soft-Ball Stage: Continue stirring and cooking the caramel until a candy thermometer reads 243°F (soft-ball stage). This takes about 30–40 minutes.
Add Vanilla: Remove the saucepan from the heat and stir in the vanilla extract until fully incorporated.
Pour into Pan: Pour the caramel into a well-buttered 13x9 inch pan, ensuring an even spread.
Set Overnight: Leave the caramel uncovered at room temperature overnight to allow it to set.
Cut and Wrap: Once set, cut the caramel into bite-sized pieces. Wrap each piece in cellophane (parchment paper tends to stick).