Familiar pie spices -- cinnamon, nutmeg, and fresh ginger -- give the pureed filling unmistakable autumnal flavor. Reroll the pastry scraps and cut out leaf shapes, baking them on their own and scattering them across the top for an effect mimicking nature's seasonal shift.
Crust: Preheat oven to 375 degrees, with racks in upper and lower thirds. Pulse flour, cornmeal, salt, and sugar in a food processor to combine. Add butter and pulse until pea-size clumps form. Whisk together egg yolk and 2 tablespoons ice water in a small bowl. Drizzle over flour mixture; pulse until mixture just begins to hold together when pinched. (If too dry, drizzle with another tablespoon ice water, and pulse a few more times.) Transfer dough to a piece of plastic wrap, loosely cover, and press into a disk. Refrigerate until firm, at least 30 minutes and up to 1 day.
Roll out dough to a 12-inch round on a lightly floured work surface. Transfer to a 9-inch pie dish. Trim edges, leaving a 1/2-inch overhang. Fold edge of dough under itself until flush with edge of dish.
Gather dough scraps, reroll, and stamp out various leaf shapes with cookie cutters. Transfer to a parchment-lined baking sheet. Whisk egg white with 1 teaspoon water in a small bowl; brush over cutouts. Refrigerate crust and cutouts until firm, about 30 minutes.
Line piecrust with parchment; fill with pie weights or beans. Bake on upper rack until bottom is dry, 20 to 25 minutes. Remove parchment and weights; return to oven. Place sheet with cutouts on lower rack; bake crust and cutouts until golden, 10 to 15 minutes. (If edges of crust are browning too quickly, tent with foil.) Let cool completely.
Filling: While crust is cooling, pierce potatoes with a fork; place on a parchment-lined rimmed baking sheet. Bake until very soft, 1 hour to 1 hour, 15 minutes. When cool enough to handle, scoop flesh from skins. Transfer 2 cups flesh to a food processor (reserving remainder for another use); puree with milk, butter, eggs, sugar, salt, cinnamon, nutmeg, and ginger until smooth.
Pour filling into piecrust; smooth top. Bake until edges are puffed slightly and center is set, 1 hour to 1 hour, 20 minutes. (If crust is browning too quickly, tent with foil.) Let cool completely. Top with leaf cutouts and serve, with whipped cream.