Place the peeled and cubed sweet potatoes in a large saucepan and cover with cold water. Bring to a boil, reduce heat to a simmer, and cook 10–15 minutes or until fork tender. Drain well.

While the potatoes cook, roll out the all‑butter pie crust into a 9‑inch pie dish, pinch and crimp the edges, and place the filled dish in the refrigerator until ready to assemble the filling.

Preheat the oven to 350°F (180°C).

Return the drained hot sweet potatoes to the saucepan (or a large bowl), add the 1/4 cup (60g) unsalted butter, and mash with a potato masher or pass through a potato ricer until there are no large chunks.

Add 3/4 cup (150g) granulated sugar, 1/2 teaspoon ground cinnamon, 1/2 teaspoon ground nutmeg, and 1/4 teaspoon salt to the mashed potatoes and stir until combined.

Allow the potato mixture to cool until it is warm but not hot or steaming (so it will not cook the egg). Then add 1 large egg and 1/2 teaspoon pure vanilla extract and stir to combine.

In a small bowl, stir together 2 teaspoons baking powder and 3 tablespoons whole milk until blended. Add this mixture to the sweet potato filling and mix until smooth. If you prefer an extra-smooth texture, press the filling through a fine sieve.

Remove the chilled pie crust from the refrigerator, pour the sweet potato filling into the crust, and smooth the top.

Bake in the preheated oven for about 50 minutes, until the edges are set and the center is only slightly jiggly, or until the internal temperature reaches at least 175°F (80°C).

Transfer the pie to a wire rack and let it rest and cool for at least two hours before slicing.

Slice and serve, optionally sprinkled with a little additional ground nutmeg or cinnamon.