How to Make It
Make the crust: Preheat oven to 325°F. Whisk together flour, cocoa, granulated sugar, cornstarch, and kosher salt in a large bowl. Melt 1/2 cup plus 1 tablespoon of the butter in a heatproof bowl. Pour melted butter over dry ingredients, and stir with a spoon until mixture forms rough clumps. Using your fingers, continue kneading mixture until no loose dry ingredients remain.
Spread flour mixture out on a parchment paper–lined baking sheet. Bake in preheated oven until fragrant and dried out, about 20 minutes, stirring once after 10 minutes. Transfer baking sheet to a rack, and let cool completely.
Transfer cooled crumbs to a food processor; pulse until finely ground. Melt remaining 3 tablespoons butter in a heatproof bowl. Pour melted butter over crumbs in food processor; pulse until crumbs are evenly moistened. Scrape crumbs into a 9-inch pie pan, and press evenly over bottom and up sides. Bake at 325°F until crust is set, about 12 minutes. Let cool while you make the filling.
Make the filling: Combine brown sugar, cornstarch, and kosher salt in a medium saucepan; whisk to combine. Add egg yolks, and whisk into a smooth, thick paste. Add milk, and whisk until smooth. Heat over medium, stirring constantly. Cook, stirring often, until mixture bubbles and thickens into a pudding, 8 to 10 minutes. Remove from heat, and whisk in peanut butter and vanilla until smooth. Immediately scrape filling into cooled crust, and spread filling evenly, smoothing the top. Let pie stand while you make the ganache topping.
Make the ganache: Place chopped chocolate in a small heatproof bowl. Bring cream to a simmer in a small saucepan (or microwave in a heatproof bowl). Immediately pour hot cream over chocolate, and let stand 1 minute. Using a spoon positioned in center of mixture, stir slowly until ganache emulsifies into a shiny sauce. Pour sauce over top of pie; pick up pie and tilt it gently to allow the ganache to evenly cover the filling. Sprinkle peanuts over pie, and chill until filling and ganache are set, at least 2 hours. Before serving, sprinkle pie with sea salt. Serve cold or at room temperature.