Make the pie crust. Combine the flour with the salt. Cut in the butter using a pastry cutter or food processor, mixing until it resembles coarse crumbs. Add the ice-cold water one tablespoon at a time, mixing just until the dough comes together. Shape into a disc and wrap the dough in plastic wrap. Chill in the fridge for at least 30 minutes or overnight.
Preheat oven to 425 degrees F.
Dust a clean work counter with flour. Remove the dough from the fridge and roll it out to about a 12-inch circle. Transfer the crust to a pie pan, trim the excess dough, and crimp the edges. Use a fork to poke holes in the bottom of the crust. Beat together the egg yolk with water and brush the egg wash on the bottom crust. Bake for 10 minutes. Remove from the oven and let cool completely.
Meanwhile, make the butterscotch. Combine the brown sugar with water and salt in a heavy bottom saucepot. Bring to a boil over medium heat and continue to cook for 2 minutes. Add the heavy cream and simmer for another minute, stirring constantly. Remove from heat and whisk in the butter, vanilla, and bourbon. Let cool slightly.
Prepare the filling. Whisk the eggs until frothy, about 1 minute. Add the sugar and cornstarch and whisk until smooth. Add the buttermilk, nutmeg, and butterscotch, mixing until well combined. Pour the filling in the pie crust and bake the pie for 40-60 minutes or until the pie is set and no longer jiggly. Remove from the oven and let cool completely on a wire rack.