Make the dough: Combine 1 cup flour, sugar, and salt in a food processor; add cold butter cubes, breaking apart into individual pieces. Pulse until butter is the size of fine pebbles, about 6 pulses. Transfer mixture to a large bowl, and toss in remaining ½ cup flour using your fingertips. Sprinkle 1 tablespoon ice water over mixture; gently mix using your fingertips. (Avoid overmixing.) Add remaining water, 1 to 2 teaspoons at a time and up to 6 tablespoons total, mixing in the same manner, until the mixture is hydrated and crumbly.

Press and squeeze the dough into a smooth ball. (Do not knead.) Transfer dough (including any bits left in the bowl) to a large piece of plastic wrap. Flatten into a disk, and cover with plastic wrap; refrigerate for at least 2 hours, preferably overnight.

Unwrap chilled dough, and place on a lightly floured work surface; lightly flour the dough. Roll out to a 12-inch circle (about ⅛ inch thick), rotating dough a quarter-turn occasionally to ensure even rolling and as uniform a circle as possible. If any cracks appear, press them closed with your fingers. Fit dough into a 9-inch metal pie pan; fold overhanging dough under itself, and crimp along the rim. Prick bottom of pie shell in several places with a fork; refrigerate at least 1 hour or up to 24 hours.

Preheat oven to 350°F with rack in lower third position. Spray a large sheet of aluminum foil with cooking spray; line chilled pie shell with foil, greased side down, allowing excess to extend over the edges. Loosely curl the foil overhang around the pan rim; fill pie with weights, and set the pan on a baking sheet. Bake crust, rotating pan halfway through baking, until lightly golden and just set, 40 to 50 minutes. Remove foil and pie weights; continue to bake until crust is golden, 10 to 15 minutes. Cool completely on a wire rack, about 1 hour.

Make the filling: Preheat oven to 350°F with rack in lower third position. Whisk together cornmeal, flour, and salt in a small bowl. Whisk together peanut butter and melted butter in a large bowl; whisk in brown sugar, eggs, vinegar, and vanilla until combined. Whisk cornmeal mixture into peanut butter mixture.

Place baked piecrust on a baking sheet; pour in filling. Bake, rotating pie halfway through baking time, until filling is evenly puffed and the center slightly jiggles, 35 to 45 minutes. Cool completely on wire rack, about 4 hours.

Make the fudge sauce: Whisk together corn syrup, ¼ cup water, sugar, cocoa, butter, and salt in a small pot over medium-low. Bring mixture to a simmer; add chocolate, and cook, whisking frequently, for 2 minutes. Remove from heat; whisk in vanilla, and let cool until slightly thickened, about 1 hour. Drizzle fudge sauce over cooled pie. Serve slices with remaining fudge sauce.