Preheat the oven to 350°F. Cut the butternut squash in half lengthwise and scoop out the seeds. Place the squash cut-side down on a baking sheet and bake for 45–60 minutes, or until tender.
Let the squash cool for 30–60 minutes. Scoop the squash from the shell and place it in a food processor. Add the butter and blend until smooth and pureed.
Transfer 2 cups of the butternut squash puree to a large mixing bowl. Add the brown sugar, granulated sugar, eggs, evaporated milk, and vanilla extract. Stir until thoroughly combined.
Stir in the salt, cornstarch, cinnamon, and nutmeg until just combined.
Place the pie crust in a 9-inch pie pan and crimp the edges as desired. Pour the filling into the prepared pie crust.
Bake at 350°F for 45–60 minutes, until the edges are golden brown and the center is still slightly jiggly. If the crust begins to brown too much, cover the edges with aluminum foil halfway through baking.
Allow the pie to cool for at least 2 hours before slicing and serving.
Notes
Refrigerate the leftovers covered for up to 5 days. If the crust is getting too brown while baking, cover the crust with aluminum foil halfway through the bake time to keep the crust from getting too brown. This pie can easily be made the night before serving it.