Grease and line a standard loaf pan with two strips of parchment paper to cover the bottom and sides.
In a medium-sized saucepan over medium-low heat, melt the butter.
½ cup unsalted butter
Add the brown sugar and corn syrup and cook until dissolved. Stir occasionally.
1 cup + 2 tablespoons dark brown sugar, ½ cup light corn syrup
Stir in the sweetened condensed milk and increase the heat to medium.
7 oz sweetened condensed milk
Stir constantly until the mixture reaches 250F on a candy or digital thermometer.
Remove the caramel from the heat and stir in the vanilla and whiskey until fully combined.
2 teaspoons vanilla paste or extract, 1.5-2 oz bourbon whiskey
Note: the caramel will bubble up and steam at this point.
Pour the caramel into the prepared loaf pan and rest for 5 minutes, then sprinkle the flaky sea salt generously over the top.
2 teaspoons flaky sea salt
Refrigerate uncovered for one hour.
Use the parchment paper to remove the caramels and place on a cutting board.
Using a large, sharp, thin knife to cut the caramels into desired shapes.
Note: Separate each piece immediately after cutting or else they will meld back together.
Wrap each caramel in 4x4" squares of wax paper, parchment paper, or cellophane.