For the Creme Filling:
In a medium sauce pan, add milk and cornstarch to create a slurry. Whisk until combined. Add the rest of the ingredients and bring to medium heat. Whisk frequently until creme filling becomes thick and begins to bubble, about 5-10 minutes. The more whisking, the better. If the bottom of the pot is slippery when stirred, use a heat-safe rubber spatula to scrape the bottom and sides of the pot into the mixture.
As creme filling begins to bubble, begin to whisk constantly. Cook until very thick, about 5 minutes.
Take off the heat. Allow to cool slightly (5-10 minutes). Whisk occasionally to make sure a film does not form on top. After filling has cooled, add it to your pie crust. Cover directly to the top of the filling. Allow to chill in the refrigerator for 2 hours or completely cooled and set.
For the Caramel Sauce:
While the pie filling is setting, let's make the caramel sauce. in a medium pot, add sugar and water. Make sure to use a pot that is bigger than what you need. Place the pot on medium to medium high heat. Stir the mixture to help dissolve the sugar.
When you see litle bubbles forming at the bottom of your pot, stop stirring completely. Allow the sugar mixture to boil for about 5-10 minutes, or until the sugar becomes a dark amber. If your stove top has a hot spot and heats one spot faster than the others, feel free gently swirl the sugar to prevent burning. Do not stir.
Once you have the dark amber, take your sugar off the heat. Slowly add in your cream, whisking constantly. Rapid boiling and rising of the liquid is normal; this is why we use a bigger pot!
Add in the butter, bourbon, and salt. Whisk until smooth.
Place in a container and allow it to cool. The sauce will thicken as it cools.
When Ready to Serve:
Add sauce to the top of the slices or the the whole pie, if it is all being served out at once. Sauce left on the pie for an extended time will soak into the pie and it won't look as nice. Consume and enjoy the fruits of you labors!
You'll probably have caramel sauce leftover. Feel free to use this on top of ice cream, over french toast, or anything else you would pour sugar syrup on.