Preheat the oven to 350°. In a food processor, pulse the graham crackers with the granulated sugar and cinnamon until finely ground. Add the butter and pulse until the crumbs are evenly moistened. Press the crumbs into the bottom and up the side of a lightly buttered 9-inch glass pie plate. Bake for 10 minutes, until the crust is lightly browned. Transfer the crust to a rack to cool completely.
Poke the sweet potatoes all over with a fork and microwave at high power for about 5 minutes, until just barely tender. Transfer the potatoes to the oven and bake for about 10 minutes, until they are tender; let cool. Scoop the flesh into a food processor and puree until smooth.
In a small bowl, sprinkle the gelatin over the 1/4 cup of water and let stand until softened. In a medium saucepan, heat the half-and-half just until small bubbles appear around the edge; cover and keep warm.
Meanwhile, in a medium heatproof bowl, using an electric mixer, beat the egg yolks at high speed until thick, about 3 minutes. Add 1/4 cup of the granulated sugar and beat until very thick, about 5 minutes. At low speed, gradually beat in the warm half-and-half. Fill the saucepan with 1-inch of water and bring to a simmer. Set the bowl with the egg yolks over the simmering water and cook, stirring constantly, until thickened and an instant-read thermometer inserted in the yolks registers 170°, about 9 minutes. Add the softened gelatin to the custard and cook for 1 minute, stirring until dissolved. Remove the bowl from the pan. Stir in the pureed sweet potatoes, 2 teaspoons of the vanilla, the pumpkin pie spice and the salt.
Put the egg whites and the remaining 1/4 cup plus 2 tablespoons of granulated sugar in a medium heatproof bowl. Set the bowl over the simmering water and cook, stirring, until an instant-read thermometer inserted in the whites registers 160°. Remove the bowl from the pan and using clean beaters, beat the egg whites at high speed until stiff and glossy. Using a large rubber spatula, fold the egg whites into the custard until no streaks remain. Pour as much of the custard into the pie crust as will fit (there will be some left over) and refrigerate until the top is just set, about 15 minutes. Spoon the remaining custard on top and refrigerate the pie until very cold and firm, at least 8 hours.
In a medium bowl, using an electric mixer, beat the heavy cream with the confectioners' sugar and the remaining 1/2 teaspoon vanilla at medium-high speed until firm. Spoon the whipped cream over the pie, dust with cinnamon and serve.
The pie can be prepared through Step 5 and refrigerated for up to 2 days.
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