Preheat oven to 300°F. Line the bottom and sides of 9x13-inch baking pan with parchment paper, leaving overhang on all sides.
In the bowl of a stand mixer fitted with a paddle attachment, combine butter, sugar, vanilla and salt. Add flour and mix on low speed until everything is combined. Press the mixture evenly into prepared baking pan. Bake for 15 minutes, until dry to the touch. Reserve for second baking with filling.
Increase oven temperature to 350°F.
Pecan Hazelnut Topping
In a medium bowl, combine nuts and brown sugar. Set side.
Pumpkin Pie Filling
In a blender or food processor or large bowl, combine all filling ingredients and blend until smooth.
Evenly pour filling over the baked shortbread base and smooth top with a spatula.
Bake at 350ºF for 20 minutes. Sprinkle reserved pecan hazelnut topping an bake for additional 15 - 20 minutes, until pumpkin pie filling has set.
Remove from the oven and cool for at least 1 hour at room temperature. Lift the parchment out of the pan using overhang, and transfer onto a cutting board. Cut into 12 to 16 bars or squares.