Step 1: Preheat your oven to 4000F (2000C). Pierce the sweet potatoes several times with a fork. Place them on a baking sheet and bake for 45-60 minutes, or until very tender. Let them cool slightly, then peel and mash until completely smooth. You should have about 2 cups of mashed sweet potato.
Step 2: Reduce oven temperature to 3500F (1750C). If desired, blind bake your pie crust: prick the bottom with a fork, line with parchment paper, fill with pie weights, and bake for 10-15 minutes until lightly golden. Remove weights and parchment; let cool.
Step 3: In a large bowl, combine the mashed sweet potatoes, softened butter, granulated sugar, brown sugar, cinnamon, nutmeg, and salt. Mix with an electric mixer on medium speed until smooth and well combined.
Step 4: Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and buttermilk until just combined. Do not overmix.
Step 5: Pour the sweet potato filling into the unbaked or blind-baked pie crust. Place the pie on a baking sheet.
Step 6: Bake for 50-60 minutes, or until the edges are set and slightly puffed, but the center still has a slight jiggle. If the crust edges begin to brown too quickly, loosely cover them with aluminum foil.
Step 7: Remove the pie from the oven and cool completely on a wire rack for at least 3-4 hours, or preferably overnight, before slicing and serving. This allows the custard to set properly.