Prepare the Shortbread Crust: In a large bowl, whisk together the flour, powdered sugar, and salt. Cut in the cold, cubed butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Press the mixture evenly into the bottom of a 9x13 inch baking pan.
Bake the Crust: Bake the crust in a preheated oven at 350°F (175°C) for 15-20 minutes, or until lightly golden. While the crust is baking, prepare the pecan pie filling.
Make the Pecan Pie Filling: In a medium bowl, whisk together the brown sugar, melted butter, heavy cream, eggs, vanilla extract, and salt. Stir in the pecan halves or pieces.
Pour Filling Over Crust: Once the crust is lightly golden, remove it from the oven. Pour the pecan pie filling evenly over the baked crust.
Bake the Bars: Return the pan to the oven and bake for an additional 25-30 minutes, or until the filling is set and golden brown. Let the bars cool completely in the pan before cutting.
Prepare the Bourbon Caramel Sauce: While the bars are cooling, make the bourbon caramel. In a medium saucepan over medium heat, combine the butter, brown sugar, heavy cream, and salt. Bring to a simmer, stirring constantly, and cook for 3-5 minutes, or until the sauce has thickened slightly. Remove from heat and stir in the bourbon, if using.
Drizzle with Caramel and Cut: Once the Pecan Pie Bars with Bourbon Caramel have cooled completely, drizzle the bourbon caramel sauce evenly over the top. Cut into bars and serve.