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To make the crust: In a large bowl, whisk together the flour and salt. Add the butter pats, tossing them in the flour until well coated. Work the butter into the flour by pressing the pieces between your fingers, flattening the squares into big shards and continuing to toss them through the flour as you work, ultimately creating pieces about the size of peas.
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Make a well in the center of the flour mixture. Start by adding 3 tablespoons ice water, mixing with a fork or bowl scraper. Add more ice water 1 tablespoon at a time if necessary, continuing to mix just until the dough comes together. As the dough becomes cohesive, you can knead it a few times to make sure it's evenly moistened, but it's important to not over-hydrate it; it should never be sticky. It should hold together in a ball, but still feel almost dry to the touch.
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Form the dough into a disk and wrap it in plastic. Chill for at least 30 minutes, and as long as overnight. When you're "ready to roll," remove the dough from the fridge. If the dough has been refrigerated longer than 30 minutes, let it rest at room temperature for 10 to 15 minutes before rolling, to allow the butter to soften up a bit.
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Preheat the oven to 425°F, preferably with a baking stone on the bottom rack.
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Roll the dough into a circle about 12" in diameter. Place it in a pie pan, and tuck the excess dough around the perimeter under itself, working all the way around and pressing lightly to help the dough "seal" to the outer edge of the pie pan. Refrigerate for 15 minutes. Crimp the edges as desired.
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Prick the chilled dough all over with a fork. Cut out a piece of parchment or foil slightly larger than the pan, place over the dough, and fill with pie weights or dried beans or raw rice.
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Bake the crust until the edges barely begin to turn golden, 15 to 20 minutes. Remove the pan from the oven, remove the liner and weights, then bake for 5 to 7 minutes more, until the middle no longer looks damp. Cool the crust while you make the filling. Reduce the oven temperature to 375°F.
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To make the filling: In a large bowl, whisk together the eggs and sugars. Add the vanilla, melted butter, and cream and whisk to combine. Mix in the sweet potatoes. The filling should be smooth but thick. Stir in the spices.
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Pour the filling into the cooled crust, smoothing the top with a spatula. Bake until the filling is set at the edges but still slightly jiggly in the center, 40 to 45 minutes. Remove from the oven and cool completely on a rack.
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To make the topping: In a small, heatproof bowl, sprinkle the gelatin over 1/3 cup (75g) of the water. Let soak for at least 5 minutes.
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Grease a silicone spatula and an offset spatula with nonstick spray; set them aside.
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In a medium saucepan, combine the remaining 1/3 cup (75g) water, sugar, molasses, and corn syrup. Bring to a boil over medium-high heat, stirring only at first to dissolve the sugar; after that, don't stir. Attach a candy thermometer to the pot, or have a digital thermometer handy.
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Place the egg whites and cream of tartar in the bowl of an electric mixer fitted with a whisk attachment. Whip on low speed until frothy. While the sugar mixture cooks, liquefy the gelatin mixture over a hot water bath, or for 30 seconds in the microwave.
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Boil the sugar mixture until it registers 240°F on the thermometer. At that point, increase the mixer speed to medium-high and add the syrup to the egg whites in a slow, steady stream. After the syrup is added, pour the melted gelatin and vanilla into the egg white mixture. Whip on high speed until the mixture is bright white, fluffy, and aerated — the mixture should hold medium peaks.
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Use the greased silicone spatula to transfer the marshmallow to the top of the cooled pie. Working quickly, use the greased offset spatula to spread the marshmallow around, making peaks and valleys.
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Let the marshmallow set at room temperature for 5 minutes. Use a kitchen torch to toast the surface, if desired.
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Unless you're serving the pie right away, refrigerate it; tent it with plastic wrap, using toothpicks to keep the wrap from sticking to the topping. For best results, bring it to room temperature before serving. Refrigerate any leftovers.
