How to Make It
Prepare the Extra-Flaky Crust: Whisk together flour, granulated sugar, baking powder, and salt in a medium bowl. Cut in butter using a pastry blender. Stir together vinegar and 1/2 cup ice water; drizzle over flour mixture, and stir lightly with a fork until flour is moistened. (If dough seems dry, add ice water, 1 to 2 tablespoons at a time, until a small piece of dough mostly holds together when slightly pressed.)
Turn dough out onto a lightly floured work surface, and gather into a tight mound. Using the heel of your hand and working from side to side, smear dough by pushing away from you a little at a time and working your way down the mass of dough, creating flat layers. Once complete, use a dough scraper to bring both short ends of dough up and over, folding so ends meet in the middle and creating 2 layers. Repeat smearing-and-folding process once.
Divide dough in half; shape each half into a flat disk. Wrap disks in plastic wrap, and chill at least 1 hour or up to overnight.
Remove chilled dough disks from refrigerator; let stand at room temperature 10 to 15 minutes. Place 1 dough disk on a lightly floured surface; dust top of dough with flour. Using a lightly floured rolling pin, roll disk into a 12-inch circle (2 to 3 inches larger than pie plate and about 1/8 inch thick). Repeat process with second disk.
Fit 1 piecrust into a 9-inch pie plate. Trim dough to allow about 1 1/2 inches of excess to extend over sides. Reserve scraps. Fold dough edges under and crimp. Cover with plastic wrap, and chill at least 30 minutes or up to 3 days, or freeze for up to 1 month.
Cut designs, using small leaf-shaped cookie cutters, from reserved scraps of dough and remaining dough round. Place leaf cutouts on a parchment paper-lined baking sheet, and chill 30 minutes.
Preheat oven to 400°F. Line chilled pie shell with aluminum foil or parchment paper; fill with dried beans or pie weights. Bake in preheated oven until edges are very lightly browned, 10 to 15 minutes. Remove foil and beans; return to oven. Continue baking until lightly golden, about 10 more minutes. Cool completely on a wire rack, about 30 minutes.
Brush leaf cutouts with egg; sprinkle with demerara sugar. Bake at 400°F until golden, 8 to 10 minutes. Set aside. Reduce oven temperature to 350°F.
Prepare the Filling: Whisk together all Filling ingredients in a large bowl. Pour into cooled crust. Bake at 350°F until Filling is firm around edges but still jiggles slightly in center, 45 to 50 minutes. Cover edges with foil after 35 minutes, if needed, to prevent overbrowning. Cool completely, about 3 hours. (Filling will continue to firm up as it cools.) Decorate with baked leaf cutouts.