Prep the potatoes: Peel and cut sweet potatoes. Boil or roast until tender, then mash until smooth.
Preheat oven: Set to 175°C (350°F). Place a baking sheet in the center rack.
Make the filling: 1- In a bowl, whisk mashed sweet potatoes, melted butter, and both sugars. 2- Add cinnamon, nutmeg, ginger, and salt. 3- Then whisk in the egg, milk, and vanilla until silky.
Assemble the pie: Pour the filling into the crust. Tap gently to remove air bubbles.
Bake: Bake for 45–50 minutes, until edges are set and the center jiggles slightly.
Cool: Let rest on a rack for 2 hours, then chill for at least 1 hour before slicing.