Prepare the crust: Combine flour, sugar, and salt in a food processor. Pulse until well blended, 6 to 7 times. Add oil in rounded tablespoonfuls. Pulse until oil is well blended, 6 to 7 times. While pulsing continually, add ice-cold water, 1 tablespoon at a time, processing until dough begins to clump and come together. Transfer to work surface. Pat dough together; divide in half. Shape into 1 flat disk and 1 flat rectangle. Wrap each in plastic wrap; let stand at room temperature 30 minutes.

Unwrap dough disk. Roll out on a floured surface into a 13-inch circle (about 1/8 inch thick). Fit into a 9-inch pie plate. Trim edges with a sharp knife. Prick bottom of dough a few times with a fork; chill until ready to use. Unwrap dough rectangle, and roll out into a 1/8-inch-thick rectangle (about 14 x 8 inches). Using a sharp knife, cut 9 (1/3-inch-wide) strips from rectangle. Braid 3 strips at a time to create 3 braided ropes. Trim rough edges from braided ropes. Wrap the remaining scraps; let stand at room temperature.

Whisk together egg and 1 tablespoon water until well blended. Brush edges of dough with egg wash. Arrange braided strips along edges of pie, pinching to blend seams and pressing lightly to adhere. Brush tops of braids with egg wash. Chill 30 minutes.

Preheat oven to 375°F. Line dough with parchment paper; fill to the rim with pie weights or dried beans. Bake until pale golden around edges, about 15 minutes. Remove parchment and weights; continue baking until bottom of crust is set, 8 to 10 more minutes. Transfer to a wire rack. Cool completely, about 30 minutes.

Prepare the filling: Reduce oven temperature to 325°F. Whisk together cornstarch and sugar in a large bowl. Whisk in pumpkin, coconut milk, vanilla, cinnamon, cardamom, ginger, allspice, pepper, eggs, and salt until smooth. Pour into cooled crust. Bake at 325°F until center is almost set, 45 to 50 minutes, shielding the edges with aluminum foil, if needed. Transfer to a wire rack to cool completely, about 2 hours.

While the pie cools, increase oven temperature to 350°F. Press scraps together, smoothing to 1/8-inch thickness on lightly floured surface. Cut dough using small assorted leaf-shaped cookie cutters. Brush tops lightly with egg wash. Arrange on a parchment paper-lined baking sheet. Bake at 350°F until golden brown, 10 to 15 minutes. Transfer to a wire rack to cool completely, about 30 minutes. Before serving, arrange crust leaf shapes around 1 edge.