How to Make It
Prepare the Crust: Stir together 3 cups flour, 1 tablespoon sugar, and 1 teaspoon salt in a large bowl. Using a pastry blender, cut cold butter and shortening cubes into flour mixture until mixture resembles small peas. Using a fork, gradually stir in 8 tablespoons ice water, stirring until dry ingredients are moistened and dough begins to form a ball and pulls away from sides of bowl, adding up to 2 tablespoons more water, 1 tablespoon at a time, if necessary. Turn dough out onto a work surface; divide in half. Shape and flatten each dough half into a disk. Wrap each in plastic wrap, and chill 1 to 24 hours.
Preheat oven to 425°F. Unwrap 1 chilled dough disk, and place on a lightly floured surface. Let the dough stand at room temperature until slightly softened, about 5 minutes. Sprinkle dough with flour, and roll into a 13-inch circle. Fit dough into a 9-inch glass pie plate. Trim dough, leaving 1/2-inch overhang; fold edges under. Prick bottom and sides 8 to 10 times with a fork.
Unwrap remaining chilled dough disk, and place on a lightly floured surface. Sprinkle with flour, and roll into a 12-inch circle. Cut dough with a 2-inch maple leaf-shaped cutter. Cut dough leaves in half lengthwise. Brush some of the beaten egg around edges of piecrust. Arrange dough half-leaf shapes around edge of crust, pressing to adhere, and brush with remaining beaten egg. Freeze 20 minutes. Line piecrust with parchment paper, and fill with pie weights or dried beans.
Bake in preheated oven 14 minutes. Remove weights and parchment paper, and bake until crust has lost its raw dough sheen, 5 to 7 minutes. Transfer to a wire rack, and cool completely, about 30 minutes. Reduce oven temperature to 350°F.
Prepare Peanut Filling: Whisk together brown sugar, 2 teaspoons flour, and 1/4 teaspoon salt in a large bowl until blended. Add maple syrup, peanut butter, vanilla, and 1/2 cup melted butter; whisk until blended. Add 3 eggs, 1 at a time, whisking until blended after each addition. Stir in peanuts. Spoon peanut filling into prepared piecrust. Place pie on a rimmed baking sheet.
Bake at 350°F until center is set, 55 to 60 minutes, shielding edges with aluminum foil to prevent excessive browning, if necessary. Transfer to a wire rack, and cool completely, about 3 hours. Sprinkle pie with flaky sea salt, if desired.
Note: The intense flavor of Grade B maple syrup, usually tapped at the end of the sugaring season just before the maple trees bud, is perfect for piemaking.