Preheat your oven to 190 deg. C (375 deg. F.) Gently fit the shortbread pie crustinto a greased pie pan and bake for 10 minutes. Remove from the oven and let it cool while you prepare the filling.
Beat the pumpkin, both sugars, eggs, vanilla, and spices in the bowl of a stand mixer, or in a large bowl using an electric mixer.
With the mixer on low, slowly add the milk and ½ cup of heavy cream, beating until the mixture is smooth and velvety.
Pour the filling into the cooled crust and bake for 40-45 minutes, or until set. The pie will be slightly jiggly.
Cool the pie on a wire rack until it reaches room temperature, then chill in the refrigerator for at least 3 hours before getting ready to serve.
Once you’re ready to serve the pie, whip the remaining 3/4 cup of heavy cream until fluffy. Cover the pie with the whipped cream, then sprinkle with ½ tsp of cinnamon.