Bake pie crust according to the directions and set aside.
Preheat the oven to 375 degrees
In a large bowl whisk together pumpkin puree, butter, eggs, brown sugar, granulated sugar, and pumpkin spice until combined. Add the buttermilk and flour and whisk until combined.
Pour into the pie crust and place on a baking sheet, bake for 50 minutes.
Let cool and refrigerate for 2-3 hours.
Serve with a dollop of whipped cream and sprinkle of fresh nutmeg if desired.
Video
Notes
You can make your own buttermilk in a pinch. 3/4 cup milk and 3/4 tablespoon white vinegar or lemon juice. Let stand 5 minutes.