Place sweet potatoes into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until tender, 10 to 15 minutes. Drain well and let cool.

Preheat oven to 350 degrees F (175 degrees C). Line the bottom of a pie plate with pie crust.

Mash sweet potatoes with a potato masher or fork. Measure 2 cups of the mash into a large bowl. Mix in sugar eggs, butter, and vanilla extract.

Mix flour, pumpkin pie spice, and salt together in a separate bowl. Add to the sweet potato mixture.

Combine buttermilk and baking soda in a small bowl. Add to the sweet potato-flour mixture. Stir batter well; pour into the prepared crust.

Bake in the preheated oven until set, about 1 hour 10 minutes. Let cool on rack until firm, at least 20 minutes.