Line a 9x5 loaf pan with parchment paper.

Brown the butter: In a medium saucepan, add the butter and place over medium heat. As the butter melts, stir it with a rubber spatula to encourage even cooking. Once the butter has fully melted, it will alternate between rapid large bubbles and gentle small foamy bubbles. The butter will be ready once it turns a dark golden color and the fat solids have darkened and settled at the bottom of the pan.

To the brown butter, add the brown sugar, light corn syrup, and sea salt, and stir until fully combined, about 2 minutes.

Mix in the sweetened condensed milk, and stir frequently as it comes to a boil. Once it reaches 250 degrees F, continue cooking for 3 more minutes.

Remove the pan from heat and stir in the whisky and vanilla.

Pour the caramel into the pan, tap it a few times on the counter to release any air bubbles, and allow to set for one hour at room temperature.

Sprinkle the flaky sea salt on top, then place in the fridge to chill for at least one hour.

Use a sharp knife to cut the whisky caramels into small squares or rectangles and enjoy!