Preheat your oven to 400°F (205°C). Pierce the sweet potatoes several times with a fork and place them on a foil-lined baking sheet.
Bake the potatoes for 45 to 60 minutes, or until they are soft and can be easily pierced with a knife. Allow them to cool until they are safe to handle.
Peel the skins off the cooled potatoes and place the flesh into a large mixing bowl. Mash thoroughly or pulse in a food processor to ensure all fibrous strings are removed, then measure out 2 cups of smooth puree.
Reduce the oven temperature to 350°F (175°C).
Using a hand mixer or stand mixer, beat the 2 cups of sweet potato puree with the softened unsalted butter until well combined.
Add the brown sugar, evaporated milk, eggs, cinnamon, nutmeg, ginger, salt, and vanilla extract. Beat on medium speed for 2 to 3 minutes until the mixture is silky and the custard is light.
Place the unbaked pie crust on a baking sheet to catch any spills, and pour the sweet potato mixture into the shell, smoothing the surface with a spatula.
Bake the pie for 50 to 55 minutes. The pie is done when the edges are set and the center has a slight, uniform jiggle when moved (aim for an internal temperature of 195°F).
Remove the pie from the oven and place it on a wire rack. Cool completely for at least 3 hours to allow the custard to firm up before slicing.
Serve at room temperature or chilled with a dollop of whipped cream if desired.