Gently roll out the pie crust and transfer to a 9-inch pie pan. Press it into the bottom and sides, trim and crimp the edges, if needed, then place the crust in the refrigerator to chill while you prepare the filling.
Preheat the oven to 425 degrees F (220 C).
In a blender or food processor, combine the pumpkin purée, granulated sugar, brown sugar, salt, spices, and molasses. Blend until the mixture looks smooth and slightly glossy.
Next, warm the filling. Transfer the pumpkin mixture to a medium saucepan set over medium-high heat. Stir frequently until you see wisps of steam rising, then stir continuously until the mixture is very steamy and makes a slight bubbling sound when you pause for a second. This shouldn't take more than 4-5 minutes.
Remove the pan from the heat, then whisk in milk and cream.
In a small bowl or liquid measuring cup (I use the same one I just used to measure out the milk and cream), lightly beat the 3 eggs, then whisk in a few spoonfuls of the warm pumpkin mixture. Slowly whisk the eggs back into the saucepan with the pumpkin mixture. Finally, whisk in the bourbon and vanilla.
Remove pie crust from the refrigerator and set on a baking sheet. (The point of the baking sheet is just to make it easier to transfer the pie in and out of the oven.) Pour in the warm pumpkin mixture. Stir together the extra egg and 2 teaspoons of water or milk, then brush this mixture over the exposed pie crust, if desired, to help it develop a glossy brown finish.
Carefully transfer the baking sheet with the pie to the oven. Bake for 12 minutes at 425 degrees F, then, without opening the oven door, reduce heat to 350 degrees F.
Continue to bake for another 40-50 minutes, until the pie is set and a knife inserted into the middle comes out clean. Towards the end of the baking time, check the pie frequently and cover the crust with a pie crust shield or folded ring of aluminum foil if it looks dark brown before the pie filling is baked through.
Let the pie cool to room temperature, then store in the fridge. Serve with whipped cream and dusted cinnamon, if desired. Enjoy!