Preheat your oven to 375°F (190°C).
In a mixing bowl, combine flour, salt, and sugar. Add the cubed butter and mix until the mixture resembles coarse crumbs.
Gradually add cold water, one tablespoon at a time, mixing until the dough comes together. Flatten into a disc, wrap in plastic wrap, and refrigerate for 15 minutes.
Roll out the chilled dough on a lightly floured surface to fit your 9-inch pie pan. Transfer the dough to the pan, pressing it into the bottom and up the sides. Trim any excess dough. Set aside.
In a saucepan, cook the purple sweet potatoes until tender, then drain and mash until smooth.
In a mixing bowl, combine the mashed sweet potatoes, granulated sugar, and heavy cream. Mix until well combined.
Add the eggs, vanilla extract, cinnamon, nutmeg, and salt, and mix again until smooth.
Pour the sweet potato filling into the prepared pie crust, smoothing the top with a spatula.
Bake in the preheated oven for 35–45 minutes, or until the center is set and a toothpick inserted comes out clean.
Once done, let the pie cool on a wire rack at room temperature for at least 10 minutes before slicing.