In a heavy-bottom saucepan, mix the sugar, agave, and three tablespoons of bourbon together. Let it sit for a moment so the sugar absorbs the liquid. Heat the pan on medium high and swirl once or twice before leaving it to boil. Let the mixture boil untouched until the mixture reaches a dark amber color. It should just begin to smoke once it reaches this stage.

While the sugar is boiling (but before it reaches that dark amber stage), mix together the heavy cream, vanilla, and salt together. Heat in the microwave for about 30 - 40 seconds, or until it becomes very warm to the touch.

Once the mixture reaches that dark amber point, slowly add the heavy cream mixture and mix well. Be careful as the sugar will begin to bubble a lot once you add in this liquid.

Remove from heat and stir in the butter, fleur de sel, and remaining 1 tablespoon of bourbon.

Make an ice bath with extra ice. Pour the caramel in a jar and allow the jar to sit in the ice bath until it becomes nice and cool.

Store in the refrigerator for up to a month.