Preheat oven to 300 degrees F. Line a 9×13″ pan with parchment paper, or spray really well with non-stick cooking spray.

In a large bowl, use a pastry blender to combine the butter, sugar, flour, and vanilla until the mixture comes together. Press the shortbread dough into the pan. Bake for 28-35 minutes or until the crust is very lightly golden. Remove from oven and allow to cool completely.

Combine all of the ingredients in a medium saucepan over medium heat. Maintain the heat at medium and bring mixture to a rolling boil, stirring often. Continue to cook, stirring constantly until the mixture reaches the soft ball stage, about 15-20 minutes). Test the "doneness" using the ice water method. Once it reaches soft ball stage, remove from heat and pour the warm caramel over the cooled shortbread crust. Allow to cool for a few minutes, then refrigerate for at least 2 hours, or overnight.

Add the butter and chocolate to a bowl and use a double boiler (or your microwave at low power, 30 second intervals, stirring in between) to melt the chocolate until smooth. Pour melted chocolate over the set up caramel and smooth into an even layer. Refrigerate until the chocolate is set. Store in the fridge, covered, for up to 1 week.