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Preheat the oven to 375°F.
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To prepare the crust: Roll the pie dough into a 13" circle and transfer it to a 9” pie pan. Crimp the edges of the dough so they stand above the pan’s rim. Refrigerate the crust while you prepare the filling.
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To make the filling: Wrap each unpeeled sweet potato in foil and bake for 1 hour, until soft. To test for doneness, pierce a potato with a fork. If it pierces easily and feels soft, it’s done. If it’s still firm, bake for an additional 10 minutes. Remove from the oven and cool.
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Leave the oven on and position a rack in the bottom third.
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Once the potatoes are cool enough to handle, unwrap them and remove the skin.
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Place the potatoes in a bowl and mash by hand. Or mix on low speed of your stand mixer, using the flat beater, until mashed.
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Mix in the melted butter, then the sugar, cardamom, ginger, salt, and orange zest.
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Add the eggs one at a time, mixing until combined, then stir in the milk.
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Remove the crust from the refrigerator and pour in the filling.
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To bake the pie: Bake the pie for 35 to 40 minutes, until the filling is firm. The internal temperature at the very center of the pie should be 190°F.
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Remove the pie from the oven and cool it to room temperature; refrigerate it if you won’t be serving it immediately.
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To top the pie: Choose either the granola-thyme garnish or the homemade marshmallow spread option to finish the purple sweet potato pie. If using granola, sprinkle individual slices or the whole pie with granola and fresh thyme leaves. If using marshmallow, dollop the spread on top of the pie and use an offset spatula or the back of a spoon to make decorative swirls. Use a baker's torch to brûlée the marshmallow spread until the edges are golden brown.
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Storage information: Store the purple sweet potato pie, covered, in the refrigerator for up to several days.
