In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and powdered sugar together for 1-2 minutes.
Add the flour, salt, and vanilla, and mix until just combined.
Press the crust dough into the prepared loaf pan. Chill in the fridge for 30 minutes.
Towards the end of the chill time, preheat the oven to 325 degrees F.
Prick the crust with a fork all over and bake for about 40 minutes or until golden brown.
Once the crust is done, turn the oven up to 375 degrees F.
For the Pumpkin Pie Filling:
In a medium bowl, whisk together the sugars, eggs, and vanilla.
Add in the pumpkin puree, spices, and salt, and mix until fully combined.
Lastly, whisk in the heavy cream and mascarpone until fully combined.
Pour the filling over the hot shortbread crust and bake for about 35 minutes until sett on the edges but still jiggly in the very center.
Allow the pie to cool completely at room temperature. Then, chill in the fridge for at least two hours or overnight.
For the Swiss Meringue:
In a double boiler, heat the egg whites and sugar, whisking frequently until it reaches 160 degrees F or until the sugar is completely dissolved and the egg whites do not feel grainy.
Transfer the bowl to the stand mixer with a whisk attachment. Whisk on high speed until soft peaks form, then add the vanilla. Continue whisking until stiff peaks form.
Take the chilled pumpkin pie out of the pan and top with the meringue. Toast with a kitchen torch, if desired.
Notes
This is a great make-ahead recipe! Make the pie the day before Thanksgiving and keep it in the fridge overnight. Make the meringue the morning of and keep it the fridge until ready to serve!
Be sure to bake the shortbread crust until it's fully baked through, about 40 minutes or so. It won't bake very much when it goes in the oven the second time with the filling, and you don't want to come out with an unbaked crust.
If you don't want as thick of a crust, you can easily cut the recipe in half!
The pumpkin filling comes together very quickly, so wait until the crust has about 10 minutes or so before you make it. You'll want to pour the filling over the hot crust.